hi Tubaloth, welcome to the club.
You already have a very good buggy and you want to tune it to be a bit more, well if I was you I'd start by putting in an electronical fail safe to forcome runaways, although runaways are fun to watch they are more fun happening to someone else.
After that you'll want to beaf up the differentialls front rear and centre.
You'll probably find that the steering servo is a bit weak and slow for racing but its O.K for bashing and general driving. Change the steering servo out for a metal gear version from a good company like FUTABA.
6volt battery packs already speed things up a bit and are cheaper than servos.
Give us a hint as to what sort of driving you'll be doing and we will try to help you get up and going.
Most of the info that you'll need is already here on the site if your really interested have a cup of coffee, sit down and read whats been done already by the boys (men) here to build some of the toughest, fastest buggies around.