Before you choose the speedo...find out your track allow SINTERED rotor or not...I know some track enforeced racer to use BONDED rotor just to make it closer to the stock brushed motor (They are still faster)...verify that first.

Then...you have to ask yourself....will you use that speedo ONLY on the 4 cell 12th scale....? Like delicated on 12th scale ONLY....? If so...yes, get the 4 cell version...it was stated by NOVAK it is maximized for 12th scale. You can still use 6 cells GTB witout the fan and heatsink but you use that ONLY if you are planning to swap the speedo/motor to the TC or something else...

Overall...if you are serious about racing (not bashing) and extra $40 is not a big deal...get the GTB line instead of the entry level XBR line.

You dont need to worry about LIPO cut off since there are none 4.8V lipo for 12th scale....yet.