I've got the a606i installed on mine (think it's that model anyway!), receipts show it was also installed by Sexton's. Looking at the iginition wires, they basically did a hell of a bad cut-n-shut job on the wires, chopping them left right and centre
However, I've got the GReddy turbo timer installed no probs, very easy with the right harness. The only problem being that you have to wait for the engine to turn off before you set the alarm, which could be what they're on about.
Doesn't matter to me though, I just wanted the turbo timer as a.) an indication of how long to let it idle (and then add a minute just to be sure! ), and b.) a pretty looking gadget