10.5T in B4?
 

been offered a LRP SPX Competition stock ESC with a 10.5T motor at a good price, this has been run in a TC.

My question is would this be ok in a B4 or would i be under powered?

It depends on usage, but I'd say that 10,5 would only be suitable for very low traction conditions where quicker motors will be hard to handle. I've run the Speed Passion 9,5 turn stockspec motor in my B4, and it felt underpowered at all times except at my local track with boatloads of top dust and no rear grip.