I Can't really speak on the mach .28 but it is more likely to be a marketing ploy aimed at people that want big engines. In all reality it will probably give a little more bottom end power, no increase in top speed and an increase in fuel consumption.
Buy the .21 engined one, and save the money towards your next engine which you can make a good racing .21 which will be a lot faster than any RTR engine.
Thats my advice.
BTW the hyper .21 4 port is not a bad engine as RTRs go anyway.