The car in question is a Japanese import 2004 Subaru Legacy GT Spec B. That's a 2.0 twinscroll turbo four cylinder boxer 280bhp engine.
It's been converted by a reasonably competant BRC installer it seems, and BRC themselves do appear to be involved in trying to resolve the issues. However any extra information that might help his installer get it sorted from people with a more general LPG system background might be helpful. To our knowledge there's only one other GT Spec B with a BRC Sequent in the UK, and BRC have tried using the map from that on this car - however that other car has issues too.
Apparently he was going to take it to Simon for conversion, but they couldn't settle on a suitable date and then his clutch failed to had to put it on hold.
Anyway - here's some information from the owner:
On part or sudden throttle inputs in the lower gears the car sometimes 'jumps' then starts pulling; as if there is some sort of delay in getting gas into the car?
Low speed running can sometimes be a bit 'chuggy' for want of a better word and parking can prone to stalling. Idle is ok but I have a feeling it might be a bit low.
The car generally feel a lot more hesitant on gas (Even though in theory it should be fine as it is a higher octane than petrol in my mind) running on petrol is certainly a lot smoother; not how the install was sold to me and certainly not reflective of the price.
When the put the car onto gas it threw a CEL. I don't know a huge amount about injectors but it was actually me that suggested it was because the injectors were not receiving the inductive/resistive loads they expected to see (The BRC ECU emulates a resistive load) and they said they'd need to investigate. Having looked at the module they fitted it appears I was right.
Small update; managed to get back in touch with the installer and spend some more time trying to setup the car; it appears the Tip-In settings we leaning out the car quite a bit under closed loop conditions. Open loop the car actually seemed to run a lot better once you were up to motorway speeds but lower speeds were rather unusable. We've adjusted this down from 60% to 40% and fiddled in between and believe this is the sweet spot. As such the car now runs a lot better in closed loop conditions and runs very close to petrol in open loop conditions. Looking at the fine tuning graph the red/green dots overlap each other perfectly when pottering around and on the odd occasiona where it's been opened up for a split second.