I think if you are dead set on a trike but sold on the M8's then you should buy an M8 bike, not trike and then do this which look much more stable not too mention fun especially in the corners. This is what I will do when I decide that I want a trike and the good thing is you can keep your ride. You just convert it over.

 

A) Yes, do it. SE high flow air filter, street cannons, and street tuner (because it can auto-learn to dial in your tune and new maps can be uploaded at home w/o going back to dealer). Why HD doesn't make this just a stock setup kills me. It is what the base level _should_ be, not an extravagance.
B) I believe that technically, ANY HD MOTOR PART, added within 60 days is covered under warranty. Not just what was received at delivery. So there is time to think about it and act after you have it at home.

 

I did the complete SE Stage 1 on my 2017 Tri Glide. The Street Cannons sound nice on the M8. The SE High-Flow air cleaner provides a big increase is filter size and air intake. The SE Street Tuner is a simple tuner and will handle the SE Stage 1 upgrades nicely. Best of all, they are EPA Cerified and wont have you worrying about warranty coverage on the road.

 

That's how I feel about our M8 CVOs nothing needed to be change, I was going to leave our 15 Limiteds alone but pinging on hot days and during hard riding was the reason to do some modifications to make them run better.