Hi mtolesen,
I agree on all the above and that there are better (modern) woofers that are generally "better", but in the right context the 2225 is no slouch. For non PA I often prefer it to 2226.
What speakers are you looking at building, active or passive, budget, size limitations, amplification, room size, music preference?
It is normally a budget/goal issue. No need to push for "11" unless the chosen design and other components support that.
True the 2225 will not get you the lowest octave but can to some extend be "rescued" with DSP and to "under port" them. Limited excursion will reduce output down there though. But they are cheap, so just get more of them if the real estate permits (like Scotts example).
The 2235 will give you the low end but at a cost of midrange performance (above 400Hz). Its all compromises.
In a 4-way I would use VLF woofer designed for the very low end (2235 or alike) as the mid range above will reduce negative midrange impact from the VLF woofer (as in some of the 70's JBL studio monitors).
If building a 2- or 3-way meeting a driver/horn combo above the woofer, I would rather go for a 2225 and sacrifice the bottom end for better mids above 400Hz (assuming a normal size horn/driver with an XO of 800-1200Hz).
You can also add a rather basic VLF low frequency sub to deal with frequencies below 40Hz. If well integrated it will be difficult to detect down there and generally do not affect perceived system quality, from my experience (masking). If you XO higher at THX 80Hz or above it will. But there is no problem getting the 2225 down to 40 Hz. This would of course imply active XO.
2216 is one of the few drivers that can do both and that is available.
Scott, I see you came to the same conclusion as I in that 4" BE's on M2's need UHF. Stylish and aggressive system :-) . Cool.
There are many roads to Nirvana and that is the beauty of our hobby :-)
My 25 cents
//Rob