First off the reason why you haven't seen 4B12s as circuit cars is that it is a relatively new engine. Give it time.
Now to try and add some thought to your considerations.
I would agree with shamrocks that it's a bit of Honda "magic"
You need quite abit more than just looking at numbers to understand all of this. The topic you're seeking to understand is engine tuning which is a dark art (i.e. requires alot of reading an understanding to grasp).
You are on the right track though questioning bore and stroke.
From first principles, the displacement of the engines are dependent on bore and stroke.
Displacement = Pi/4 X bore^2 X stroke X #of cylinders
So since all engines for the purpose of this discussion are inline 4s we can adjust this formula to read:
Displacement = Pi X bore^2 X stroke
If you do this for any of the 3 engines discussed (i.e.
4B12/
B18/
K24) you will find their displacements.
so the bore(diameter of each cylinder) and stroke (travel of the piston) directly affect the displacement.
The compression ratio increases the proportion of energy which can be extracted from the detonation. Too high and you get unwanted knocking, too low and you're being inefficient as the air/fuel mix is still burning when the piston reaches the bottom of the stroke and the exhaust valve opens.
Which finally takes us to the part the manufacturers don't always discuss i.e. the valving or air flow. Notice on all three engine description pages on Wikipedia, none of them give any "real" information of the valves nor manifolds (intake and exhaust). So it could well be the shape/form and dimensions of these which make the difference.
However, you have to understand that "tuning" these parameters involves manipulating a host of factors but primarily "timing" (MIVEC or VTEC) is the main factor which affects the final figure.
Depending on the drive train warranties and the Eco reporting of each engine, engines are tuned differently coming out of the factories. Remember that the 4B12 is the newest of the lot and has to meet the MOST stringent emission standards whilst the Honda B engine was released at a time when things weren't as bad. Furthermore, most Mitsu's tend to come with 5 yr warranties whilst the Hondas only go for 3yrs. From what we know now through hacking the factory ECUs, we know that the 4B11s and 4B12s were very severely detuned. So they're letting the engines not go as hard but they are certainly capable of going much harder.
Now with tuning, one seeks to optimise valve timing in order to improve burning efficiency. This however will increase the power within a certain range and drop off power at another. So even when you go to a tuner, it would be good to see where he is getting your power. It's much easier to tune for peak power at high REVs but how usable is that? Let's now go back to the specs:
The numbers game:
4B12=
Peak power 125 kW @ 6000 rpm
Peak torque 232 N·m (171 ft·lbf) @ 4100 rpm
K24=
Peak Power: 150kW @ 7000 rpm
Peak Torque: 222 N·m (164 lbft) @ 4500 rpm
B18=
Peak Power: 154kW @ 8000 rpm
Peak Torque: 181 N·m (133 lbft) @ 6200 rpm (98 spec)
Numbers may well be larger but where is all that usable power? Don't let the numbers like kW sucker you into believing higher is better.
Looking at the numbers though, you might also want to consider the track you're going to be driving on? Twisty bendy? or one with a couple of straights so the Honda has a chance to get up to those RPM levels?
I could go deeper into the underlying theory but I suspect this will suffice.
