Having the SP model should mean that you'll not be needing the cases bored out to accept the bigger liners that the big bore requires. If it were a BP then it's an engine strip.
I've just done a lot of work and put a spec. together for my own 853 kit for the 748, and other sources you might want to look at are Ferracci, Vee Twins and JE Pistons.
The one thing I've discovered with the DP stuff is that the crank usually needs balancing due to the pistons being heavier than stock, and this adds to the cost.
I don't know the weights of the 916 pistons, but by using a piston of the same weight, you could negate the change and therefore not need to rebalance the crank.
This saves a lot of time, and time=££'s where this type of work is done.
I do know of a pair of JE 955 pistons sat on a shelf waiting for a home though.
I've not done the maths and groundwork on the conversion you're looking at, but if you'd like any info. your welcome to mail me.
FB's right about the gains. 10-15% everywhere wouldn't be an unreasonable gain to expect.
I'm sure you wouldn't regret it.
As FB said though, JHP, AJ or Sigma would be the kiddies to talk to.