I had a 2005 Beta 250 and I'm at 1100MSL. Yes the motor is soft and super smooth, but works well here, for all but the biggest stuff. I'm sure at 7000' it's a dog. I've ridden enough at 10K plus on regular dirt bikes to know how altitude really effects the power.
I bought a new GG 250 and it took me awhile to get used to the snappier power, even though it's also a 250. My guess is your bike is stock and running fine, but just weak at altitude. Sell it to somebody at seal level and buy a bigger bike.