Sorry late reply - been busy...
OE turbo is 19PSI max. anymore and you overspeed it and you actually loose power due to the heat generation - (and will damage the turbo in time)
Hybrid turbo's are a slight challenge for us - as the OE exhuast housing is really rather small and it creates too much pressure in the exhuast manifold.
Have a read of my build thread for some of the issues i have encountered using mine....
alternately nip over to mg-rover.org and read the diesel section if you want to see other
tuning options. (i dont think there are any other L-series specific sites where people are tuning - but there is always tdiclub and the forums for more generic (but VW orientated) tuning options)
Or feel free to ask me here or pm