I may be alone on this one, but I've had bikes (96ci 6spd) with stage one and with only slip ons. I can't feel a difference, except for the heavier wallet when I go with just slip ons. I'm sure there's a difference, but I'll be damned if I can feel it. I did the stage one on my 08 Road King, but went with just slip ons for the 09 Street Glide. It won't break any speed records, but then again, that ain't why I buy Harleys.
In most cases your bike will run smoother, in some cases it will run a little cooler, and in a few cases it will get better mileage, but in all cases it will sound like a Harley-Davidson Motorcycle and make you feel good when riding your Harley.
every bike is different and every dyno is different but with a decent tuner and some dyno time you should get 10-15 more hp and tq. a stage 1 with the hd download only will get you half that at best....
I was very dissapointed in the performance of my stock 09 UC. It sounded like it was bogging down when opening the throttle all the way. Heat was bad on the right side. I added Rush slipons, SE AC and stage 1 download. I was so impressed that I canceled the Pro tuner that I had on order. 14,000 miles and I am very happy with this bike. It runs circles over my 06 UC with PCIII. Mileage is still around 40 mpg. About 32 mpg on my 06.