would getting a set of webber carbs for the A12 make any difference to performance? i'm not after a race engine just a bit more power

Without a doubt, more fuel plus more air equals more go.

what's the whole deal going to cost? roughly because i'd seriously consider doing now that i've got a little cash

You wont notice any real differnce on a stock motor without other upgrades. Just bad fuel economy. Your money would be better spent on a weber 32/36 downdraft, or maybe even a single sidedraft. But for money go the downdraft. They feed more fuel and air than a stocko engine will ever need. Complement it with an exhaust and you'll have a nice free revving engine. Spend wisely and you'll end up with a much better performing car then spending everything just on carbs.