how long have you had the engine and how many miles have you done on it? when a motor is run in you do get a hike in power - two in the case of the GEBE engines apparently.. this might be why you have a lower top end..
Is the engine tuned correctly - does it seem to suddenly loose power at a given engine speed? that might have something to do with it...
have you checked that everything is running freely? the more mechanical drag on the drivetrain (ie badly adjusted brakes etc) there is the harder the engine works and the less speed you will have.
being that this is a friction system is there a problem with drive slip? - if that is the case then that could lose you more speed and after a while will mince your back tyre.
there is also the issue of all up weight - if the machine itself is heavy that will drop speed.. ie I would get more speed on a racer bike (up to the point it shredded itself or I ended up buried in the side of something executive) than I do on a MTB frame - and given identical machines someone at 140lbs will have a slightly higher top end than someone at 230lbs.
and finally - are you on the same size roller etc, that will change the ratio of the 'final drive' as it were and mean different performance for different setups...
I'm running a 1.5 inch roller (few hills) and I weigh 135 dripping wet. I took it pretty easy on the motor 'till I had about 200 on it (not really, but I tried). Also, I'm running a City Slick on the front and a pretty slick Armadillo on the back. Both with 60+ lbs air. I disassemble, clean, and White Lithium grease every bearing surface on the bike annually. Finding the exact pressure of the roller on the tire is also important.
I'm gettin old, For me it's not ALL about the speed. It's about being able to go places these old legs won't easily get me. And not paying much for it (relatively).