Oh man...this could be an insane list of things...mostly because I have probably tried more products than anyone else here that doesn't own a company. A lot of it was learning experiences...but some were just bad decisions and having a little extra coin in my pocket.
Couple of things I personally wouldn't do again:
Big Wheels - going above 20 in the rear really made the ride suffer and made the slingshot very heavy to drive.
Klockwerks windshield - Overpriced and not much difference from the stock one...especially at higher speeds.
Factory SL windshield - Overpriced and didn't give me the air flow that I wanted
Graphics - I love what was custom made by the folks at AMR racing...but I should have really considered a full on paint job.
Sparco Steering wheel - Should have found a smaller one...feels like I am driving a school bus at times. Also I can't seem to line up the steering wheel where it's actually level when I drive.
SSV sub box - simply an awful product compared to other sub boxes on the market.
I do plan on doing some changes before summer...assuming everything goes smoothly with the new home and new baby on the way. New Spoiler, Powedercoat the current wheels, and need to get some of the tuning corrected.