John...
Modifications I have seen (mostly read about on the Pelican Parts Forum)..
Barrier Hoses
This seems to be the most recommended option. Original hoses are fail in a predictable fashion by having pinholes in them. Apparently this means they will leak over period of time and R134a will leak more quickly than the original freon (r12). By using barrier hoses the aircon keeps the gas in for longer and therefore works at its optimum for longer. Worth doing but a lot of work. Costs can be reduced if you can get someone to make up the hoses using fittings from the old ones but where in UK?
Keule Condensor
Mounts in left rear wing. Has its own fan. Used in addition to standard front and rear condensors. Supposedly makes quite a difference. Some hove voiced concerns about having a device intended to remove heat mounted so close to the engine/exhaust. Overkill for UK?
Underbody Condensor
Mounts under the body of the car. Exposed to the elements but it is a specially toughened coil so can withstand a good bashing from road debris. Again supposedly works well but not so great when stationary as no fan. Overkill for UK?
964/993 Condensor
One or even two of the 964/993 condensors mounted in left and right front wings. Bit sketchy on this but I think they have fans so should work well. A lot of work though with quite a bit of relocating of existing parts and fabrication involved. No kit for this that I know of. Again, overkill for UK?
Various replacment compressors
More effiecient compressors available. Not sure that any real benefit to the aircon but they take less power and are quieter.
Receiver/Dryer
Should allways be changed when system has been exposed to air. Possibly everytime you recharge. There is a version called the Procooler which claims to improve the performance of the system. Some debate over its effectiveness.
Evaporator
Have seen a drop in replacement for the evaporator on US models (with eveporator in the smugglers box). Claims to be more efficient due to different coil design. No idea what model evaporator is in the UK versions. I think its a Behr of some sort.
One of the features of the condsensor mods is that they add additional airflow by using fans. Sounds like you allready found that out. Obviously all the mods add additional condensor area but most of the guys that have fitted these seem to live in areas with much hotter temperatures than the UK.[

] - How' s your' s coping today!
Cheers
Andy