my bot was running on 4s mainly at cheltenham and portsmouth, and throughtout the testing phases between, all were ok. I went to 5s on 2 fights at cheltenham, and yes there was an obvious increase of speed, i ended up reducing top speed on my tx range as i found it a little too uncontrollable, especially when turning on the spot.
So it is a toss up on how experienced a driver you are compared to top speed. i think 4s is a good start and maybe progress to 5s when you have had experience driving that particular bot.