Appreciate the input.
I want to try my hand at the 'cheap' upgrade first - either the 2.3 or the 2.5. The 2.5 isn't completely off the table. I see that they come readily in the Mazda 6 as well, which makes it even more easy to get. The 2.5 gets us up to 125kW/229Nm using the factory tune. With a bit of intake/exhaust tweaking, tune and perhaps a CAM we should be able to get this up closer to the WT Fiesta ST non-overboost in the short term. PLUS, the Manual WQ is 100kg lighter than the Manual WT.
I've also uncovered a post of someone advising that the L5 2.5 has internals almost as strong as the MPS 2.3 engine, with forged crank and rods at least - which keeps the future open. The pistons might be the limiting factor. Some notes on the 2.5 MZR engine:
MotorReviewer 5L-VE summary
MotorReviewer 3L-VDT Summary
Thread on 5L-VE internals
So to the big question - is certification of a 2.0L to 2.5L swap in NSW a big deal?
As for the gearbox, I see that the IB4 was only mated to the Duratec for the XR4/ST150. Everything else used an MTX75. Google says that there were some kits a few years back for fitting the MTX75 manual trans into the ST150 - but I don't see anything still current. I understand it needed custom transmission mount, changed gear lever cables and custom driveshafts. Finding anyone with either these kits or the specs for these kits would be a bit of good luck.
Lukeyson