Did I notice a drop, no. Was there one, theoretically...yes. However I never really cared about top speed myself.
The RTX came stock at least one if not two teeth lower geared on the top sprocket compared to the GT.
Going from a 121" to a 144" most people would gear down I assume, but I left mine as is when running a 1.25" lug track.
There are a lot of people more knowledgeable than me on this site that could either support or shoot down this theory, but if you gear down, it is possible you could get more top end. While it seems counterintuitive, there are trains of thought that say by gearing down, you aren't lugging the motor and transmission as much, and it allows you secondary to shift all the way out giving you the highest possible ratio. I know I read somewhere that most factory sleds are over geared from the factory and never achieve full clutch shift. I have never dug into the theory on this personally, but it makes sense to me. I think you will tend to see the topic of over shifting/under shifting come up more on the mountain forums. In that arena, track speed is everything.
I hate to tell people this because I think there is a bad stigma that goes with the title, but I am a degreed mechanical engineer. Far more relevant than that is I call myself a glorified grease monkey. I've turned wrenches most of my life, and the hands on experience is what I tend to dwell on mostly.
I should point out I had two drive axles. one with 9 tooth extroverts for when I run flatlands with a 144" ripsaw, and one with 8 tooth extroverts I would throw in when I did a track swap. I had a custom mount plate assembly I designed and fabricated that allowed me to to set back my suspension. When I went out west I ran a 151" x 2" Camoplast series IV track.
