You shouldn't have problems with 28 spline with 270rwhp. 35 spline is overkill and will cost you a small fortune because everything but the housing will need to be thrown away. 35 spline only come in detroit locker (fairly rare) and full spool types, so it isn't really suitable for street use anyway.
Traction, heavy cars, and a lot of HP is usually what breaks 9 inches. In other words, slicks on a race track. Even then, it's usually only manuals that have troubles. I have a mate with hundreds of 9 second passes on 28 spline and 800hp (its an auto, and now 31 spline)
I've been through a heap of broken centres. I've twisted axles (31 spline) but never broken any. That is with launching at 6,500rpm with a toploader and a fair dose of HP.