Well, a 225/55-16 has a rolling circumference only about 1.2% greater than your stock Metz or Avon 240/50-16 and about 2.5% less than the Bridgestone Exedra Max 240/55-16. In other words, the difference is negligible while the tyres are upright and rolling on their full circumference.
However, because an MT has a curved cross section, the sidewall height is about 2-2.5 inches less than an equivalent sized car tyre. This means that when it leans over in corners, the actual rolling circumference of the MT drops markedly - by about a foot (in fact, if the motorcycle tyre were to run fully on the very outside corner of the tread, it would be closer to 18 inches less). Fortunately, because the front tyre is similarly curved the relative circumferences of the two tyres will be more or less constant within a few %.
It's difficult to estimate how much a CT distorts when it leans, and it will depend on inflation levels, but I doubt the sidewall squashes down by 2-2.5" (unless it's dangerously underinflated) so it would be reasonable to assume that its rolling circumference in a corner would remain closer to what it would be when vertical, therefore throwing the relationship between front and rear out significantly in corners. I would imagine this could interfere with the ABS if the tyre sizes are too far from stock size. With the 225 rear, a 140 front would JUST be within reasonable bounds. Personally, if you're a fastish rider and like to push the bike through corners, I'd recommend sticking with a 150 unless you're going to also run a lower profile rear.
On the plus side, with a car tyre on the back, when cornering the rear can lift relative to the front somewhere approaching 2 inches so this could make the turn in feel more responsive, once you got used to the transitions and the constantly changing geometry.
The above effects may in reality not be all that noticeable to the rider but I'd wager the ABS is more sensitive. I don't believe I could say with certainty which tyre sizes would be OK, but it's pretty obvious that if you were to run a 245 CT on the back, you'd be more likely to have problems with ABS and most likely have to stick to a 150 front.
All in all, it still seems an odd thing to do to an otherwise precision piece of machinery. You can call that prejudice if you like but it's really just mathematics - and a preference not to make my Rocket any more difficult to ride than it was designed to be.
