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You would be surprised at how much skirt can come down before there is an issue. Some of the blocks we have done get the pistons going WAY down in the bore. +11mm stroke Hayabusas come to mind. 9mm of their 27.5mm skirts will be below the bore. Not a small amount, and yet they live and make SERIOUS power. Its all about getting the clearances right.
2.5mm leaves it awfully thin in my opinion for something that is expected to live a good life. A 'hours' engine, sure, go for it. Not a good option for the street. Stock or 1mm over should work well.
The 250 block is the way to go for BIG pistons. Just remove the stock sleeves, bore out the block, put larger sleeves in, bore the upper case to match the new sleeves, and bore the sleeves for the new pistons. Thats 45 year old tech. Simple and cheap. Downsides are it will cast just as much to do it for the first time as just having a Nikasil bore done, after that its stupid cheap. It wont last as long between having to re-ring and de-glaze. Heat transfer sucks with iron sleeves compared to aluminum. Nikasil wont be as cheap but the material is MUCH more durable, and heat transfer is substantially better. Not to mention its a lighter block...
I really wouldnt bother with extra CCs, as there isint a LOT of power to be found there. Its all about the head and cams, and getting the lower end to last.
-Mike
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