DJ - this is very interesting, and I've read your posts on Seloc too - well done for trying something different. Some of my thoughts:
You could add another engine steady in the same way that the Honda conv's have one to the front - quite easy to do and would take the strain off the driver side engine mount - which you can fracture with a standard k.
Regarding the exhaust fabrication, you can buy preformed 90' 3" stainless bends (what custom exhaust manufacturers use) to help keep the flow smooth.
I'd have expected that mounting the turbo under the ex manifold (similar to Simba's) would help keep the heat away from the boot space, and the 3" exit could flow down from the turbo and under the engine - keeping the flow neat and helping keep radiated heat in the bay and against the firewall down.
On a standard 111S, heat is transmitted through the firewall after a good blast or prolonged driving - handy in the winter but not the summer - this just highlights the need for good thermo/heat management. A chimney blowing from below will just blow more heat onto the boot - that will be a bout 4-6" away from the turbo, so a jacket would probably be a nessesity. Alternatively, if you don't yet have the rear clam - get an exige (or copy) - as this has a higher boot.
Something like a 2bular GT3H2 single exit is around 8kg, the cat is around 1-2, so this would save about 15-20kg's over the original S2 111s exhaust and cat. Not quite standard, but you could use a dummy left hand exit if you want. Most people wouldn't notice the differennce though.
Regarding your charge cooler - how are you ensuring the charge is cooled? Is this watercooled via a jacket? Did you consider a fan assisted intercooler on D/S wheel arch? what is your reasoning - I may need to reason along similar lines.
As for the extra weight over standard, you may want to call Gaz Pro and discuss what rate of springs you should use on the rear to make sure everything is as it should be. You could get it corner weighted too once its all done.
As for KingK's and other comments, well each to their own for engine choice. What you are doing with the K is great, and people choose Honda, Audi, Toyota, GM, Ford or Rover turbo's based on their sitution and what they want. I'm sure that they weigh up the pro's and cons, and they pay their money and takes theirs choice.
I deliberated over tuning the K, (but dodgy heads, mass HGF, reliability and cost with no guarantee of longivety put me off), the Audi was perhaps too torquey and heavy (plus not openly discussed or available mounts) but the TFSI other than weight seems very tempting other than cost.
For me - the Honda fitted my spec. Yes it was a little heavier than the K, but not 100Kg heavier like the Toyota cars are (yes thats chassis modifications too), and yes I can feel the added weight in the back more than I thought, but I put this in for £5.5-6k, and that includes lighter flywheel, baffled sump, 2bular exhaust and 100 cell cat, MSC 4-1 mainifold, adjustable FPR, custom shafts, kpro ecu and a tune to 235-240bhp - but I did the build myself.
Although its heavier, its more reliable, and a good basis to super charge or turbo to 350-450 bhp as a winter project. I'm not sure how much a KingK motor is, but to be honest, I'd not heard much about your motors, and pondered a DVA 195 build and dismissed it.
DJ is sticking to a powerplant he is familiar with and has contacts to help complete the build at a very keen price that would be hard to beat - I personally welcome any comments, as it makes for more of a debate, and we may learn something, so long as we dont thread drift too far.
I bet you can't wait to test drive your creation, just be careful this time of year - those roads are slippy as hell and full of grit, and you will accelerate faster than you can brake with those skinny front tyres and no weight on the front - but any thing over 200bhp in 800kg feels great, so 350bhp is gonna feel epic.
Silly grin time