u could custom a supercharger or a turbocharger.. it might cost u around 10k as well (u need the turbine/compressor, piping, intercooler, engine management/piggyback, etc)
as for the roadtax, 3.0L costs a bit higher than the 2.5L but fuel consumption does not differ much (I own an A32 3.0L).
but if ur after performance.. fuel, modifying costs are EXPECTED to be high..
if ur lookin for performance, there's no replacement for displacement..
if u want cheap performance mod.. put in a 2.5 inch exhaust system, a less restrictive air filter and a fuel management piggyback..
power ain't cheap bro..