High Sensitvity = good transient response ?

high sensitivity usually = poor bass.

Marakenetz - is your response "high sensitivity usually = poor bass" a horns thing, or are you also referring to cone transducers as well - I am on the horns track, fwiw - and you're definitely correct there (the physics are damning)... but I've never actually scratched my head about any cone transducers claiming anything above 93db. Always given that territory to horns. An honest "I'd like to know more" here.
If anyone knows more, and has a moment, let's hear it!

It is possible to achieve good transient response and have high efficiency in a bass driver. Trying to achieve great extension AND high efficiency is a tough thing to achieve though.

The problem with most high efficiency designs in terms of bass reproduction is that they are of some type of vented design. Vents are always slower than an optimally tuned sealed box with a reasonable Q. Since sealed boxes are typically inefficient, there are few that achieve this. One exception is the Klipsch Heresy, which is sealed, has pretty quick and punchy bass ( good transient response ) and is relatively high efficiency ( 96 dB's ). Only problem is that in stock form, it is rolling off pretty hard by about 50 Hz.

Bass horns tend to add their own colouration unless phenomenally well constructed. I've never seen a mass produced model that worked really well from the factory ( in terms of "hi-fi" ). Sean
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Again, we arrive at the inevitable conclusion, every design has its flaws.
For Mwilson, I have been using Lowther fullrange drivers recently and they have 98db/1watt efficiency unloaded, 100db/1watt in a cabinet. In my cabinet, they are good down to the low 40's, and they have a resonant freq of 36, so I am not "on the spike", but rather rolling off a few db above it. This leads to a rather steep low cutoff which you can either live with, or integrate a very good sub. Right now I am living with the 40Hz bottom end, and it is quite enjoyable, although there are some records that display the flaw. But, the natural tonal quality, and speed, detail and phase coherence makes up for the 1 bottom octave of missing bass response. I know some disagree with this thinking, but few audiophile speakers actually produce much below 40Hz without some rolloff. So the sound is not really lacking much compared to most others. I have toyed with the idea of integrating a pair of TL subs of my own design, but haven't done it yet. If you have not listened to some Lowther systems, then you should really try to find some to audition. A quite remarkable full range driver.

As as been alluded to, there is a tradeoff relationship between efficiency and bass extension (box size also factors in). It is possible to build a high efficiency system that truly goes deep, but the required box size is pretty big.

There also seems to be a correlation between efficiency and dynamic contrast.

Lately I've been playing around with vented bass systems in the range of 95-98 dB efficiency. By using an unusually large enclosure and tuning it very low, you can get transient response that rivals a fairly low Qtc sealed system. This approach is used with great success by Classic Audio Reproductions, and my own results haven't been too bad. Sean, you might want to try running a few simulations with, for example, a TAD 15" woofer in an 8 or 9 cubic foot vented box tuned to somewhere in the lower 20's. Compare that curve with a Qtc = .6 sealed box for the same driver, and you'll see the potential of this approach.

One of the reasons you typically see high-efficiency speakers using vented enclosures is that the driver parameters conducive to high efficiency (low Qts, medium to high Fs, and large Vas) really aren't suited for other enclosure types. Very low Qts woofers are suited for horn enclosures, but now we're talking refrigerator-size cabinets.

An example of the opposite extreme is the Carver Sunfire line of subwoofers. Carver has chosen to go with an extremely inefficient design (and equalization) in order to get deep bass in an incredibly small box. That's why the Sunfire has such an enormously powerful amplifier.