Lazer ZX-5 + brushless: which gear ratio?

[korez]

Hi mates,
I've a Kyosho Lazer ZX-5 RTR and I'm interested about Losi Xcelorin brushless combo (combined with Trakpower LiPo 2S 4900mAh 25C), particularly the 4.5T: it's a good choice in your opinion? Or the 6.5T is better? On 4.5T the original gear ratio will be ok or I must replace it with new and different spur and/or pinion?
I'll bash on 1/8 buggy tracks.
Thanks!