if your after the 250 bhp mark i'd get rid of the piper and put a newman set up in instead and if you go zvh and skim the pistons like you say then you will be limited to the boost these will handle due to inherent weakness of the thinner crown and heat build up. if you go the cvh route then the rods and pistons would be close to there limits and it's anyones guess as to how long they would take the abuse for. if it was me i'd build the cvh for now and over time build a propper cvh with all the goodies in i.e forged pistons and steel rods as a true 250 bhp road car is plenty and will surprise a few people and with the propper bottom end goodies in place there is nothing to stop you upping the boost for a few more ponnies but don't forget the gearbox and clutch while the engine's out.