Quick look at motortax.ie assured me I've made the right decision with a 2.5 (vs bigger engine) I couldn't bring myself to pay anymore than 2.5 tax. It would ruin any enjoyment the car could ever provide to be honest.
http://i1350.photobucket.com/albums/...ps4b15265d.png
Having said that, track car with stupidly large engine for weekend blasts would be nice though :D