The STS D30M is a rocket plain and simple. BUT it does have it's draw backs. First the carb o-rings suck, then don't seal very well and allow the needles to move around making tuning very frustrating. Second the needles themselves have a very course thread so a very small turn will result in a large change in air fuel ratio.
If you are patient with the needles and get a good set of o-rings you will be all set.
I have an RB modded STS D30M and love it, BUT I also got an Axial .28 Spec 1 RR and let me tell you it rips. I would say the .28 RR is the fastest .28 I have ever seen, including the LRP Spec 3 which is another engine that I love. If the STS's issues have you second guessing the .28 RR is amazing. Tunes just like a stock K4.6 with about twice the power, or so it seems in my Hellfire.
To compare the standard Axial .28 to the STS .30 is a little unfair. The Axial is a novice race engine and the STS is an expert race engine. Give both to the locals at the track and most of them would get better performance out of the Axial because it is more user friendly. The STS is the faster mill, by far, but because you need to be ontop of your tuning game to get the most out of it 80% of the people racing for fun would miss the power tune.
If the STS is in your future run 30% nitro fuel and a hot plug. I run Trinity monster power 30% and a McCoy MC-59 plug. The 30% will lower your temps and help with the break in. I suggest a heat cycle type of break in. I posted my break in plan here,
https://www.hpisavageforum.com/forums/showthread.php?t=2799 . My plan is a very long winded ordeal but the end produce is a very fast well running mill.
Good luck with which ever mill you buy.