True, but the engine has less character and a narrower power band (even remapped). However, the standard chassis and brakes won't take much more power than the 2.4 delivers; a remapped 1.9 might also be better in that regard.
From my experiences I've found the 1.9 to be a bit of a beastie when remapped. There's even someone on here with a 250bhp 1.9 JTD..
I'm not sure about the chassis not being up to it, but I'd be a lot more worried about the transmission.. Didn't Hunter have quite a lot of problems with gearboxes, driveshafts, turbo, etc on his remapped 20v 156?
Also the 20V 156 gearbox is not Q2 compatible, the 1.9's gearbox is. That and the lighter engine would swing it for me in favour of the 1.9, and you know I'm a big fan of the older 2.4 10v..
As for brakes, unless its the GTA they need to be uprated, even if its just fast road pads and discs and some good fluid.
Sounds like the engine might be my compromise point then...
I don't think you could go wrong with either of the diesels, but the 20V's are quite a bit more expensive (but usually are better spec'd). I think if it were me, I'd spend a bit less on a 16v and use the rest of the cash to bring it up to spec.