OK, well, in order of payback/effort, I would rank my changes as follows.
1) front wheel bearing spacers. Large improvement in straight line stability in return for about a day spent tinkering on a lathe cutting a little off at a time until bearing clearance was correct.
2) the plywood splitter under the nose. £6 worth of 6mm marine ply, a length of PVC drain pipe to make the blend piece and about a day to fit it all.
3) the all-steel steering UJ, with the proviso that you check the internal splines are properly formed, mine weren't.
4) the 2psi residual pressure valve in the brake line, cured all that uncertainty with a long brake pedal travel. Nice and hard now.
5) modern tyres on 15" wheels. Helps over in UK that there is a plentiful supply of MGF wheels, and they chopped and changed designs a lot, so you're not forced to go with one style. I chose the one that looked nearest to the Elise wheel. Big increase in grip.
6) creating spigots for wheel location. Wheel vibration now respectably controlled. Not easy to achieve however, I've attached a picture of front and rear spigots. The PIA factor comes in setting the spigot suitably concentric tot the hub, about 0.001" TIR. Then you realise every time you disturb them, the same procedure has to happen all over again. Also found balance shops don't necessarily set the wheel to the spigot bore, so I made my own static trimming mandrel, see pic. One of the reasons why this is a long way from my #1 recommendation, it needs tools and time. Maybe one of those ebay balancers might work, I thought my answer was better.
7) rear discs, quite a lot of work, but the car needed a bit more rear brake and in these days of all the other cars having ABS, I didn't want to get caught out running into the back of someone in an emergency braking situation, front wheels locked and smoking. Especially true in the wet.
The VVC engine BTW is great, but massive effort and only for the really committed. The Renault XF swap was a weekend job IIRC, as compared to 2-3 months for the VVC motor.