How big a job ? not as bad as you'd think. Can get big if the crank needs balancing. Getting pistons that match stock weight is the key to preventing this.
Reliability issues ? shouldn't be any. the bottom end is pretty strong.
Additional service costs ? none really.
Why do it ? makes a great engine. 748 buzz with 916 grunt, or more if you spend further on gasflowing, cams etc
A models that cant be converted ? No, it can be carried out on any of them really. The R makes a superb engine.
Benefits ? see "why do it"
Cost ? £1000 ish upwards depending on spec and if crank needs balancing etc.
Can it go back to a 748 ? Yes. If the crank is rebalanced due to new pistons, then it's a little more difficult.
A job you could do ? I can do it, yes
916 barrels/pistons were/have been used. 2 problems, the barrels are taller and need machining and the pistons are heavier requiring crank balancing.
Sigma's kit uses stock weight pistons, as did my kit. I'll probably not go the route of having any more pistons made as the cost, quantities etc. make it financially difficult having them sat on the shelf for some time.
I'd probably use Pistal pistons, or Sigmas kit.
I'm hoping to get the fuelling sorted on mine soon and see what it's really capable of producing. It had the legs on 999's at brands, but was only doing about 18mpg, the top end is very rich, so there could be more to come
It's a bit more than just a set of big pistons though