The question Psag poses could be taken, or rephrased, either of two ways: 1) Is there any advantage in going to biamping (by doubling up on identical amplifiers) if the single amp already in use is high-powered to begin with and the speakers are of average difficulty to drive (and let's also stipulate that the room size and listening volumes desired are around average)? Or, it could be taken to say: Is there any advantage to biamping (using identical amps) over just single-amping using a similar model (let's say from the same manufacturer) but whose construction and output specs are nominally twice that of the smaller amps?
Or put most simply: For an average system and listener, is there nearly always an advantage (in absolute terms and cost-effectiveness aside) in biamping where the power is doubled, regardless of how high it was to start with -- and is there an advantage in biamping where the power remains constant?
Putting aside for the moment the additional and important questions of whether we are talking about "passive" or "active" biamping (and assuming we don't screw it up if it's the latter), and also whether the amps are stereo or monoblocks (not to mention whether the cables in the starting condition are single- or bi-wired), based on my experience (again, limited!) I'd say the answer in the first instance would probably be yes -- even if the original amp or amps are high-powered to begin with and the demands are average, there may very well be an absolute advantage to be gained by doubling-up (amps + power). But then again, I'm an advocate for the notion that, other things being equal (which they won't necessarily always be), more power is better.
The second instance I can't address from experience, but I'd hypothesize that yes, even though the total power doesn't increase, there could still be some advantage to be gained in divvying up the duties so that the HF amp or amps don't 'see' the LF driver(s) -- much the same way most audiophiles believe that a pair of monoblocks can exceed a similar stereo amp of equivalent power per channel in part because an amp dedicated to each channel doesn't 'see' the demands of the other channel.
So, my opinion taken to its logical conclusion: In a multiway system, the best scenario would be to have an independent mono amp dedicated to each driver (I personally don't, and likely won't, have this), and you basically can't have too much total power, even with 'average' speakers and room/listening demands. Fire away...