After going through a lot of literature I can to the conclusion that all this carb boring isn't all that difficult, if you have access to a lathe and bear a few things in mind. Interestingly enough, the throats are already at 33 (and a bit) mm, but the backplates are choked to 29mm.
Next step is to put the backplates in the lathe for boring them out to the same diameter as the front. Now the only thing you have to bear in mind is to bore the carb eccentrically, as you can see, I did this with a few strips of copper sheet wrapped around one of the jaws on my lathe. If you bore it centrically (which is of course possible and would yield larger gains in terms of maximum flow) you have to lower the needle jet and recut the grooves in the carb body that make the slide seal at the bottom. Furthermore this would really upset the idle mixture, so basically it's not really worth it...
And this is the finished product. Yes, it did work but the gains weren't as drastic as I had hoped. I should have offset the carb less and bored it in a wider circle as I ended up with an oval carb.
(By the way something Mikuni did in the early eighties on a round-slide carb intended for the Suzuki DR500!)