Installed a pair of Polk DB652's in fairing yesterday. Install is smooth if you have the swivel adapters to get the socket to the 3 problem bolts on each side and the 2 Allen head screws on each inside of the speaker pod to that mounting bracket.
As well, ensure you disconnect any plug that gets in the way of the socket extension so you don't break any OEM plugs.

NOTES:
1. Included flat mounting rings are not needed and if you try you'll find the speaker pod will NOT fit as the adapter hits the sides of the inner fairing.
2. The OEM speakers screws are not long enough with these new speakers. But, if you have longer ones I think the holes line up correctly and you can use the original holes.
3. I just turned the speaker a bit until I had enough room to drill 4 new holes into the pod. Just ensure there is enough room to drill all 4. The supplied screws needed are the longer ones in the package with the 8 screws and 8 triangle nuts. Not sure what those things are really called.
4. When you drill new holes, just drill smaller than the new screw diameter. The plastic will heat up when you screw in the screws and when it cools down the screws should stay locked in. I used the 4 mounting holes on the new speaker that are open ended, NOT the other holes as they are too close to the speaker cone.
5. Also, the new 8 screws WILL screw down thru and into those open ended holes so be careful not to screw them too far in, OR, use a small washer on each to prevent this. I screwed them in just till they were tight and the speaker didn't move.
6. The OEM speaker connectors on the speaker end are backwards from the Polk. The OEM has a small connector as positive(+) and the larger one is negative (-). I had some flat male wire spade terminals and used them to connect to the "included" speaker wires with connections. I just had to cut some of the width from two of the flat spades to fit into the OEM small connector. This way I didn't need to cut into ANY OEM wiring! You could also just buy a set of spade wire connectors, just make sure it has the small one's included.
7. New grills won't work either. They are made to screw in from the outside, while the fairing grills are connected from the inside. I left the OEM grills on and did not notice or feel any restrictions when putting the pod back on.

**There may be grill adapters that will work but I have not seen any google results showing which,if any, will work correctly for the Polk DB 652's into a 2015 Street Glide Special.
The Harley Stage 2 adapters might work. I know the Stage 2 speaker grills are on the outside of the fairing, but I haven't seen how they look like on the inside, so I really have no idea.
Worse case, you leave the OEM grills on, and stick the supplied POLK labels onto the OEM grills so it doesn't look so much like the OEM.

I did notice a difference/better sound from OEM to these. There not going to blow you away or anything unless you add an amp. The HU is only 25w/ch., with I believe maybe gets you around 12w rms. Don't quote me on that!
So of course not pushing a lot of power to the speakers will affect the sound and you may notice distortion/clipping at higher levels. But in my case they are plenty loud with my Cobra "neighbor haters" slip-ons. Matter of fact, some songs on my USB card are not as good quality as others (some have lower volume, lower bit rate etc.,). The quality recordings really DO SOUND great even without a external amp to my ears, and I'm in my 50's so what I hear is different than what you may hear, which is normal. That's why I get a kick out of every time I see someone ask what speakers etc., sound better..

Bottom line, I spent $59.95 (free shipping NO Prime) on Amazon for these filled by Amazon, shipped from CA and not the other sellers. I have read that there are other people selling Polk speakers that may/may not be true Polk speakers.

The next objective I need to tackle is whether or not I need to modify the OEM speaker pods? Again, I cannot find any information if someone already did it this and it would need to be on an identical bike/speaker-install.
I've built a sub woofer box before so I understand the requirement of having enough free air space to allow the speaker to properly work.
But on these pods, I'm not going to start cutting ports into them for more air flow unless I find some information on the subject. Putting too much free air space would be the same as not having any enclosure.

And finally, I'm not sure it's even needed! I don't recall reading anything about Harley Stage 2 with speakers and amps requiring ANY modification to the OEM speaker pods.

The biggest hurdle with putting aftermarket speakers in the newer batwing fairings is that the speaker surround will touch the inner fairing. Most people use hogtunes grills because they protrude out more to allow room for tweeters but are also thinner on the mounting flange. Make sure you have enough clearance for speaker movement.
As far as porting, you can try it if you think the speaker is restricted but my guess is that it’s not needed with that set up. You could drill a few 1/2” holes in the back and see how much air moves, tape over if it isn’t needed.

I'll try and answer you all at once lol.
Inthewindmn:
I did not notice the speaker surround touching, the pod actually went back on as easy as the OEM did. Actually, I can push in on the middle of the grill and not touch the tweeter unless I push in about 1/2" or so, so I think I'm ok. But now you have me curious so I may take them off to see if I can tell anything touches and/or do some measuring. I could just take the grill off, put pod on, and see if there's any sound difference and compare.

Tailwind:
It was costs for me. I'm self employed and $$ is not just flowing all week long lol. Plus I had read many threads and these were one of the choices, or more like one of the better choices unless you have a lot more $$ to spend.

travelingypsye
Yes I can hear them fine. I could hear the OEM one's fine too, they were just flat sounding. Guess it all depends on your exhaust and how damaged your ears are. I've never needed to push the volume past the last 3-4 bars even up to 80mph.