I have a Wimberley AP-601 replacement foot on my 400 f/2.8, and I plan to do the same with my 100-300 f/2.8.
They are the smallest replacement foots (at least for Canon) out there, after comparing with those from RRS and Kirk. They help me save so much space when packing (I used to attach a multipurpose rail from RRS to the original foot).
I wish Wimberley had QD sockets. It seems like the QD system is pretty convenient.
I don’t really travel with these lenses (only local/driving use) so packing space isn't an issue for me. The RRS replacement feet aren't a problem for the 600/4 going in the big Lowepro Lens Trekker 600 AW II, nor for the 100-300/2.8 going in the Think Tank DH150 toploader-style case. Another concern is being able to comfortably use the foot as a handle for carrying the lens, including wearing gloves in winter with the 600/4. The RRS replacement foot is good in that regard. How is the Wimberley for that?
The QD system is very convenient. The LCF-53 foot for my 600/4 predates it, but the LCF-53B foot that I got for the 100-300/2.8 has the QD socket (as far as I can tell, that's the only difference with the 'B' version). I did not replace the foot on my 600/4 with the B version because the strap I use with it is the Blackrapid Sport-L, and that doesn't come in a QD version. The -L is for left, it's designed to hang the rig on the left side of the body and that's ideal for the 600/4 where I prefer to lift it with my left hand, since that's going to support the lens as I shoot anyway.
Not sure what strap(s) you use, but prior to the QD system (and currently for the Sport-L with the 600/4), I have a little Kirk 1" clamp attached to the strap lug (with a bit of Loctite Blue on the threads). That allows easy removal of the strap or switching the attachment from body to lens. It's just as convenient as the QD system, the only drawback is that it's a bit bulkier.