I rode it today GPS doesn't lie ever think you talk too much? I'm out didn't think trying to help a guy decide on a good ski would turn into a pissing match with you.
I tend to agree with R2B on this, if your gtx 185 does 68 on gps, then either
a) you have the fastest stock gtx 185 on the planet
b) your gps is wrong.
c) ???
Perhaps there is a C out there that i'm not aware of, but gun to my head if somebody asked me top speed on a gtx 185, then I wouldn't hesitate to say 62-63 absolute MAX. (actually my first instinct would be to say 60, but i'm adding 3mph for leeway)
now, I still don't think its underpowered, but everyone is allowed an opinion, and I'm ok with his opinion that a buyer should hold out for a 215/255 model for long term happiness. I don't think its a must, since many many people would be very happy with a 155/185 model, as I mentioned above, i didn't cross 60mph until I had been riding for years, but keep in mind, back then, 2 strokes were more prevalent, these days 60 is pretty slow depending on who you ride with.
Given the choice of the two listed by the OP: I chose the 07 155 rxp over the 04 185 gtx because I'd expect they'd perform very close overall, and the rxp is a bit more fun to ride. the only caveat I added was that if its going to be regularly used with passengers, then the gtx is more practical, it will comfortably hold two riders (3 is a stretch but do-able) and has tons of storage.