I'v herd of a few people doing this with there cars, are there big benifits and is it costly

reboring is mainly used for rebuilding when you have scorn the cylinder, you could oversize to any given amount and is only limited by the cylinder wall thickness. as for performance well a 253ci block bored out 60thou with over sized pistons ends up at around about a 265 or there abouts which is a increase of around 4%. you can go further but manufacturers dont really make pistons for their cars past 40thou now days and therefor you would need new pistons made from billets.

best off with forced induction unless your limited by CC due to rally rules and regulations. stroking is similar again and gains are very expencive unless there is mass produced kits which lower costs.