Well after finding out the hard way the other day that my ST205 was limited I decided (after much research) to go for the standard HKS SLD as my clocks are already UK and reading in MPH. Spent a good few hours hunting around for installation instructions and was pointed in various places. All in all was a tiny little bit tedious.

So, for the benefit of others:

ECU will have 3 plugs. What you need are:

Big Brown Wire (Earth)
Black/Yellow Wire (12 Ignition Power)
Orange Wire (Speed Signal to ECU)

Check an ECU pinout diagram if you're not too sure where those are but they're all unique from what I can remember and pretty easy to find imo.

Make sure you have a decent set of crimps or needle nose pliers as the bullet crimps supplied aren't the best for the size of wire you'll be crimping, ended up soldering mine.

Black Wire on SLD -> Brown Wire
Red Wire on SLD -> Black-Yellow Wire
White Wire on SLD -> Orange Wire Loom Side
Yellow Wire on SLD -> Orange Wire ECU Side

There may also be a blue wire on the SLD but this is not used from what I can tell. Instructions are only guaranteed for ST205's and not other GT4s as I can't honestly say everything will be the same, I would imagine they are but without having wired one up in one I can't say for certain. SLD also tucks away nicely under the little L-Bracket thing under the ECU that the carpet velcro's onto. Works a treat as far as I can tell. Hopefully this information will help someone.