Noice!
What sort of diff do they have in these things?

Generally they are open differential unless optioned up.  Being a Cup 260 variant this comes with a Torsen differential of which make I would only be guessing but I guess a reasonable educated guess would be a Quaife unit, considering both Quaife and Lotus and UK brands.  Not completely sure though, was the C64 ever offered in any toyota cars with a torsen LSD?

-Totenkopf-

robk
In terms of drivability, the post-throttle superchargers make tuning the ECUs on the SSC Lotus cars more difficult, I know this for a fact! It takes a significant amount of time for the engine to consume the post-throttle air when the TB closes, which basically adds a few complications which don't usually exist.

Any ideas for some workarounds rob or things I can try considering this thing has an adaptronic ecu essentially.  I don't see why the throttle body couldn't be moved post charger on the manifold and then vacuum line for the supercharger bypass (SC is always spinning so it bypasses air when there is a vacuum on the inlet side of the charger caused by a closed throttle butterfly, essentially allowing the SC to freewheel and consume little power) sourced post TB from the manifold, though I am no SC expert.  If there is an ECU work around that can improve the situation this would be much better though I cant think of much thats reliable.  Maybe at revs above 2000rpm with the clutch depressed (assuming there is a clutch switch, else fit one) you could apply ignition cut so that the engine isn't still trying to produce power when you go to shift gears.  
 
Obviously it would be good for me to confirm it is actually the supercharger causing the issue and not something to do with DBW throttle.  Hopefully there is some logging functionality in the ECU so I can log throttle percentages vs boost pressure and see if pressure remains after throttle is removed.

I'm not much of a supercharger expert either unfortunately.
There were some settings added to the Adaptronic ECUs specifically to help SSC with the Lotus plugin units. TBH, I can't remember what the settings were though, and maybe they were added after my time anyway.
Like I said in my PM, go on the Adaptronic forum (http://adaptronic.com.au/forum/) and ask about it on there, or email tech@adaptronic.com.au, and you will probably get a very thorough response within 24 hours. Adaptronic has seriously awesome support. Andy is known for coming up with clever custom workarounds to unusual tuning and/or drivability problems.

Had a look on the forum there already after I noticed a large plenum charger option in the software, looks like this should help with the situation, I'll see if its been enabled on my car soon.  My trusty old laptop is pretty rooted nowadays and have zero battery life or I'd have checked already.

I would not worry about the spring rates compared to the Sw20. The MR2 has a 1:1 ratio as its a mac strut, the Lotus has a higher ratio due to the location of the Coilover because of the double A link suspension. Could be 1.1/1.2/1.5, but it will require a heavier spring to gain the same spring rate as a mac strut set up.

Plenty of laptops down my way if need be.

-Totenkopf-
Any ideas for some workarounds rob or things I can try

Enabling decel fuel cut should eliminate this problem.  Set the threshold to say 1500RPM, or whatever is needed to recover to a stable idle.  Zero throttle, above 1500RPM = no fuel, revs drop quickly.
 
I tackled a similar issue for a completely different reason - a Fairlady Z that had a prehistoric blow off valve that vented into the inlet manifold.  Essentially it was a pressure actuated throttle bypass, and fuel HAD to be cut to make the car drivable.

cogs

-Totenkopf-
Any ideas for some workarounds rob or things I can try

Enabling decel fuel cut should eliminate this problem.  Set the threshold to say 1500RPM, or whatever is needed to recover to a stable idle.  Zero throttle, above 1500RPM = no fuel, revs drop quickly.
 
I tackled a similar issue for a completely different reason - a Fairlady Z that had a prehistoric blow off valve that vented into the inlet manifold.  Essentially it was a pressure actuated throttle bypass, and fuel HAD to be cut to make the car drivable.

Hey cogs, haven't seen your name pop up for a while!
I'd be surprised if decel/over-run fuel cut isn't already enabled in tot's ECU, but if not then yes that could explain it.

Haha, been lurking for a while!  
 
Another thought, the TPS may also need to be recalibrated so the ECU sees the correct zero point.

Baller!
 
Lost two of the MR2 crew to Lotus this year so far.
 
Oh and Rob who went over to the UK to work with them :)