Hi Ian
Theres loads of info about the older non PD units and tuning of them in the TDI section, have a read back through, probably best to start from the last page as its the oldest stuff.
but a brief answer is yes theres more that can be done but itsa more involved that the simple remap. Your looking at different injectors (transporter tdi ones), a different turbo and maybe a FMIC but its not critical unless mega power is needed.
the standard airbox for the tdi leons is very effective and no real modding is needed, but a higher flow panel filter has been known to improve turbo spool but no more power is gained.
anyway, have a read back though, should keep you busy for some time.