The thing that determines the 'peak' power is going to be the MOSET's themselves. They have a somewhat 'soft' limit. It's not like voltage, where even going 1V over might be enough for the FET to go short. You can go a few A's over and things should be OK if you keep them cool and the duration short enough.
Continuous power handling is an entirely different beast. It's really only determined by how much heat the controller case can dump. If you water cool a 18FET and do a few mods to get a really low delta between the silicon in the FET and the heatsink, you can basically run the controller at its peak power 24/7. Otherwise, continuous power can be as low as 10% of 'peak' power.
According to this spec sheet...http://www.irf.com/product-info/datashe ... 110pbf.pdf
The IRFB4110PBF TO-220's commonly used in high power e-bike controllers should be good for 180A at the silicon level, which is the more important number if you really are interested in pushing things, especially since we don't tend to be riding the current limit that long so long as it is sufficiently high. A good assumption is that for every time you double the number of FET's, you can handle 1.5x the current. This means every additional FET adds roughly 2/3 of its max rating. Since the controller only really cares about phase current, we will use that, assuming it's 2.6x higher than the battery current, at least in the lower speed region thats more critical. For an 18 FET controller, we have 3 phases, making 6 FET's per phase, which is then split up evenly into a high and low side. This means your power handling calculations should be done with the assumption of 3 FET's doing the 'work'. With 4110's, you can handle a max of 180 + 2(2/3)180 = 420A in a 18 FET controller. This is roughly 160A battery current. Setting the battery/phase to 160/420 respectively would roughly be asking the controller for 100%, which tends to be a bad idea, since this limit can often be loosely enforced at times. Playing it 'safe' would put the max limit around 20% lower than this, 125/200. This should be OK if you can keep the controller cool.
100% limits are roughly 16kW for an 18 FET 4110 @ 100V.
Moving on to the 4115's....http://www.irf.com/product-info/datashe ... 115pbf.pdf
104A silicon limited. Same 18 FET controller, 104 + 2(2/3)104 = 242A phase, 93A battery. Assuming you run 150v into these (bad idea), you can get roughly 14kW out of a 18 FET IRFB4115PFB controller, absolute peak. Take 20% off this to 'play it safe' and you are down to just over 11kW. Even trying to get 11kW out of one of these would be a struggle, but it should be possible with the right tweaks.
100% limits are roughly 14kW for an 18 FET 4115 @ 150V.
Run the 4115 at 120-130V and the limitations are even more obvious. The IRFB4110PBF's are just better FET's for power. Unfortunately >100V right now isnt the best way to get a ton of power, at least with off the shelf hardware.
I should probably note, these 100% figures are not reasonably obtainable. Running a controller even at 80% of its limits takes a mad man.