Had these thoughts myself whilst mine was stripped down in feb, and decided against the 853 in the end because of the resale side of things.
When I get to the point of needing (as opposed to wanting) more power, I'll go for the 996R/998R option. Although, if one comes along at the right time and price either
916SP3 (955cc) or 916SPS (996cc)
would do nicely.
But....whilst in MotoRapido on Friday, Steve did tell me how much it would cost to put a 999 engine in my chassis, and it was cheaper than the work I'd had done in feb, but about twice the cost of an 853.
A friend of mine had a sigma conversion on his 748 and it made 106 bhp, not bad but not really 996 territory. If you have a 748R it would be much closer and a real hoot to ride I expect, but if resale value worries you then I'd plump for the bigger bike.
True TP, true, I must admit I thought they were a bit more than that. There's not really any advantage over a good 748R there is there? AJ was saying to me that he got nearly 130 bhp out of an 853R, that would be tres cool I reckon!