Stu I also had the same thoughts as you in March this year, except I went a bought new Beta 125. Everyone thought this was a crazy idea, going from a Beta 270 to a 125!! However, I haven't ridden the 270 since! I'm 11 stone and find the 125 has plenty of power for me and believe I do ride it better then the 270. It has the racing kit fitted (available from John Lampkin) which includes different jets for the carb, a different middle box for the exhaust plus you can fully advance the timing for even more power(which I haven't done).
I am more than impressed with the 125 and would recommend one to anyone considering one!