A 5.5 in a touring car will run = to a good 8 single. On 125 foot straight in an actual race on concrete with rubbers it will hit in the mid 40s. The torque is the issue with the brushless motors.
A 6.5 is supposed to be = to a good 10 turn. Got mine today for my XXX4.
The 5800 is about like a 12D.
The 4300 is like a 19T.
For offroad 2wd I would run the GTB w/ a 5800 to start.
For 4wd run the 6.5 or if you dare, the 5.5.
If speed is all you care about and you are not going to the track then the 5.5 is the ticket.