I very much doubt a "U" can be more rigid than an "O" even if it's got lot of weldings..

hmm seems like a worth while mod!

Is it 'weldable' in situ, or does the subframe have to come off?

i had mine done last weekend, and the sections that were welded are accessible in situ but.. i would definately recommend removing the subframe, and stripping it of wishbones and arb to do this.

quite simply the metal skins not actualy that thick, and the heat from the weld travels, as i welded near where the wishbones are mounted, leaving them on would risk melting the bushes or worse..

re-assmbly is reversal of removal, but i recommend fitting polybushes at the same time

also clean as much of the collected dirt/crap as you can off the subframe, it'll probably save you a few pounds of kerb weight, for a while anyway

ninja

Perhaps so, but if you just take a look at the subframe when it's new or removed, you'll see that it's just 2 pieces of metal plates folded together and welded in only a few places. However, this U shape is not 2D but 3D, so I don't think it can be simplified by compairing an "U" to "O". Having said that, I've fitted and removed the subframe struct brace and couldn't feel any difference IMHO (after welding the subframe when it's new), that's why I removed it. No point speculating if you can actually try it