Gearing question

I'm considering to upgrade my gear ratio for faster speeds and possible competitive purposes but I don't know what ratio is optimal for my car (TB04 4wd Touring, 21.5). I read that somewhere between 3 - 5 is ideal but I don't want to overgear it. My stock gear ratio is currently 6.6 (66t Spur 25t Pinion x 2.5) and is it worth upgrading? I noticed the increase in speed when I switched from LiPo to NimH but will the same increase be noticeable with gears? Also, if anyone has experience with this specific car, what 64pitch gears can it fit without modifying the chassis (drilling, cutting etc.) ? Thanks! :)