A .28 would definitely make it faster. Sometimes too fast. Most people run a .21 in buggies and truggies, but if you want it built for pure speed than a .28 may be what you want. Also getting a clutch bell with less teeth will increase your top speed but also decrease acceleration if you make too big of a change. I run a .21 team orion engine in my buggy with a 14T clutch bell which I may change for a 13T which is what I run in my truggy which has a .21 dynamite engine. My truggy is faster than my buggy but also takes more time to reach top speed. A .21 should be plenty fast for a buggy considering its smaller size and the difference in fuel consumption. A .28 sucks up noticeably more fuel than a .21 meaning more $spent on fuel and a shorter run time. Also if your running fuel with less than 25% or 30% fuel you can get a little more performance by moving up to 25% or 30% but remember moving up to a higher nitro % means it runs hotter (leaner) and you shouldnt drop down to a lower %. Another tip dont run more than 30% nitro.