I wouldn't be too concerned, T-Rexs can be made to run very hard. I think for what you are looking for, the T-Rex was a better choice than a Cub. A Cub is what it is, a top end monster with port timings that will be marginal down low. I wans't really impressed with the one and only Cub, that I ever got to ride. It was fast, no arguing that, but the powerband just wasn't what I like my motors to be. If I had a place to ride wide open almost all the time, hell yeah, they are fast and cheap.
I prefer powervale motors above all others, I think the Cheetah would be a better choice than the Cub. Alot of people seemed to have forgoten that these exist since the Cub was born. If you look into them there isn't a huge difference in price. On paper the Cheetah seems much more expensive until you consider everything it comes with. I have seen some very fast T-Rexs, come to think of it I've never seen a slow one.