I have bought several times BTC for Euros and only standard 0.6% fee was applied by MtGox 
It would be highly unusual if they matched orders from several currencies.
IMHO the 2.5% fee applies only in case you have i.e. no EUR wallet and you send euros to their account, so they have to be converted. Or when you convert it manually. It doesn't happen at trade time.
It would be highly unusual if they matched orders from several currencies. IMHO the 2.5% fee applies only in case you have i.e. no EUR wallet and you send euros to their account, so they have to be converted. Or when you convert it manually. It doesn't happen at trade time.