Which speakers are higher efficiency?

Gotta say I agree with Johnk and Gregm on this one. You need at least 95dB to make those 300Bs work really well, and I would almost say you need 98 (though it would depend on the impedance curve (and at that level, phase angle may come into play :^) but I'll defer to others on that)). Anything below or with a large impedance range with the low impedances in the bass and you will be risking serious disappointment (especially in the bass). I think the Coincident Total Eclipse are good high efficiency speakers in the 'box speaker' format, but my heart belongs to horns, which can come in all kinds of flavors. I'd think for $4-6k you could do yourself very well, either by buying something used off the 'gon or buying speakers custom-made for you direct from the manufacturer (disclosure: I've never purchased any speakers from any manufacturer who mentions himself on this thread (or is likely to going forward for that matter) but I've seen some of his creations in photos. Other builders are those who deal with BLH (back-loaded horn) designs. You can check Frugal Horn for some DIY designs, though there are some builders who will build a set for you, sometimes ones which are tweaked/improved beyond the original design. You might also dig around the archives here and over at AudioAsylum.com (the High Efficiency Speaker Asylum). There will be lots of ideas in both places.

One of the great ironies/mysteries of life is that one usually needs more power to get full-range sound in a small room than in a big room ("but it's a bigger room so needs a bigger amp!" he said). To get full-range efficient speakers, one is usually dealing with a large box which may require more distance. Small bookshelf speakers require more power to drive well. Such, to the everlasting chagrin of horn-loving studio apartment dwellers, is life. As Johnk mentioned, the Model 15 does a decent job (though noone would ever accuse it of being a 'big-boned' bookshelf speaker), and depending on your tolerance for tall speakers, something like a Cain & Cain Abby might also work - again... I recommend a dig through the archives.