Checking back, decision made, trade done.
I ended up doing something I thought I wouldn't and now have a 2.8/28 Elmarit ASPH on the way. I will now present my logic (or lack of):
1. I like wide angles, I really enjoyed a 21-35 pair, except I didn't like the external VF with the 21, and changing lenses (and removing and installing the VF) was a big PITA.
2. I couldn't decide which 35 to go for, I had trade offers for both a v3 and v4 and I was going crazy. I also had a trade offer that included a 2.8/35 ZM, and while I'm not a big fan of the ZM ergonomics it got me thinking that I could live with a slower lens.
3. Out of the blue, an offer for a 2.8/28 Elmarit ASPH presented itself. While "researching" the Elmarit, I came across a thread here where I'd said that the 2.8/28 ASPH was a lens that I had wanted to try!
4. I hope it gives me a little more of the wide angle drama that I like a was missing from the summilux (due to long MFD, I found it hard to push the perspective).
5. I figure I can use the whole VF on my M4
6. I thought what the hell, I'll give it a shot.
So in the end I went wider, but looking back on the thread it seems I was leaning wider towards the end!
I have my hoods for the summilux, the 12504 and my hacked 12524, both for sale in the classifieds. I'll take a few rolls of film the hacked hood if anyone's interested.
Thanks to everyone who helped out, it was all very helpful (even if it doesn't feel like it at the time!)