No need to worry about the voltage - we use 4.8v in both 12th and GT10 on BL motors without any concerns. Bedworth Club have run 4.8v TCs using Bl, again with not problems.
Just because you're a but less than a volt short of the LiPo does not mean you'll be much slower. A track is 10% straight, and 90% corners and links that aren't always full throttle, and where a good chassis counts for more than pure power.
Give it a whirl, it's closer to the future than LiPo is! HTH
