I've got a 748SPS and my friend has a 748R, both with 45mm cans.
The R has more mid-range than the SPS already, so its prob the gruntiest 748 package.
If you dick with your engine size, you'll find it much harder to sell when you eventually want to.
Ditto for fitting another engine, unless you keep the old one to put back.
If you tot up the cost of an 853 kit, and a 'full-monty' type fitting job, cos whats the point if you have it all apart anyway, I think you could justify trading up to a 9xx.
I've been deliberating this all year