It's possible that you may in fact get more compression, but if the compression goes much higher than the 9.5 to 1 which I believe is stock, then you will need the more expensive premium fuel, or to retard the spark a little which will negate the reason for the compression increase.

On the upside the smaller ports & valves in a stock A12 head will help ensure better bottom end performance & will probably give you quite good mileage but just move the red libe on your tach back a bit as it won't rev as well as a stock A14 would.

But hey, it's a Ute, a workhorse that's slow anyway so the A12 head, which will fit right on, will help you to avoid any nasty old speeding tickets.