It costs a few quid to get the block correctly bored to the oversize AND of course it means the whole lump has to come out of the car (if you're just fitting standard-sized pistons, the engine stays in the car and the garage will just run a flex-hone down the bores to break the glaze)
The bigger pistons ARE better, especially if you go for the higher compression Accralites, but the associated costs are higher (I reckon you're looking at £200 minimum, maybe more, depending on the machine shop). Also, the boring requirements are VERY specific - it took quite a while to find a machine shop that could do it the way Accralite stipulated.
Yes - I'll do you a great price on the rings, but it probably wouldn't be in exchange for a decade's supply of microfibre cloths - I have plenty