I can recall seeing Ultimate Seats, TricLED, and SpyderPops doing installs of items purchased from them at Spyderfest. I also recall there being some pretty long lines of folks waiting to get stuff done, so just a heads up. Though to everyone's credit, looked like they moved through those installs pretty quickly given the space and setup constraints.
I guess the thing to consider is the trade-off....waiting to have stuff installed or going out on rides etc. The stuff you mention doesn't really take long to put on, but if its you and 50+ others having the same thing done, there is going to be some down time while you wait to have it done.
Myself, I had no problem getting what I wanted then installed it when I got home.
That's my .02