I have quite a lot of first hand experience of Efi throttle bodies

In my experience it's not only the throttle size but the taper of the inlet tract, trumpet size and radius, inlet length, injector placement that can have a dramatic effect on bhp and torque (pulse tuning).

If you go too big on the throttle size it will make the engine boggy low down and very on/off when engaging the throttle. If your after big numbers when flat out then bigger usually works.

Look at www.jenveydynamics.co.uk

They do some nice spigot bodies with a good range of air horns