I have two that I've done that to (a 360/12WB and a 330/12). In my opinion, those mods are down the list a bit in terms of capturing "the sound". This would be the order of mods or changes I would make, in order of effectiveness:
(1) Get a JangleBox. Nothing else has such a big effect on the sound.
(2) Switch to flats. The Thomastic Infeld Jazz Flats are pretty hard to beat in terms of sound, playability and long life. The rather dull tone and lack of twang of the primary strings brings out the chime of the octave strings.
(3) Start using a very thin pick and a light touch. For flat picks, I finally settled on Dunlop .46 mm nylon.
This will get you pretty darned close with no mods to the guitar. A large portion of the sound is also generated by picking through the chords and chord-to-chord transitions, not strumming. This is pretty important, as strumming a twelve generates a full sound, but one that tends to be rather feature-less.
(4) Switch to toasters - It can be done, and toasters are great pickups, but to my ears they don't make a huge difference in the sound.
(5) Add a middle pickup - but only if you have a plan to do something "different" with it, because just adding one into the system doesn't seem to do much.
I have three toasters on the 360/370 and a toaster neck pickup and two high-gains modified to take toaster covers on the 330/340. There is some difference to the sound between the two pickup types, but it's not particularly drastic, and I think it's hard to say that one is really "better" than the other. I do generally like the sound of the toasters better, but after going through the first three steps above, I think you will find switching pickups may not get you an awful lot of additional bang for your buck. From a mystique and cosmetic point of view though, toasters are clear winners (just don't expect to be able to tell the difference easily with the lights off).
I found the same to be true on the addition of the middle pickup. It's not a drastic sound change, and it certainly isn't required in order to get fantastic sound out of the guitar. I tried all sorts of stock and non-stock wiring plans with mine, and most of them would not have justified the addition. I finally settled on running the middle pickups alone, on their own circuit which can be run to a different channel, track or effects box to modify the overall sound. It is handy to have at times, but again, not at all critical for the vast majority of the stuff that I play and record. So while you may notice some differences in the sound after adding another pickup and doing some toaster-izing, it well may not be nearly as much of a difference as you were anticipating, and in terms of the cost and hassle of doing the mods, the value may be somewhat questionable compared to simply doing #1, #2 and #3 above and modifying your playing style. You also want to make sure that you're comfortable picking in a fairly small space between the pickups with three of them present. It's not particularly hard to get used to, but does seem a bit cramped at first, compared to a two-pickup model.
Sound clips - most of these are just using the standard neck/bridge, 5-knob Rickenbacker system with the middle pickup turned off. They all use the JangleBox, TI Jazz Flats, and picking through the chords using the thin nylon flatpick. There really isn't a single typical strum anywhere, in any of them.
http://webpages.charter.net/tbradshaw/M ... amples.mp3
