Well, that high a reading indicates either a refrigerant overcharge or a blockage somewhere.
Depending on the ambient temperature even the 325 that it settles out to may be too high. Did the shop tell you how much refrigerant they installed, by weight? The amount of oil sounds about right assuming there was none in the system when you started. (Normally if there is too much you would get less cooling rather than a severe increase in pressure.)
The service manual for 2000 XJ shows the following for expected pressures at different temperatures:
This gives you an idea of what would be considered normal operating pressures depending on the outside temperature.
A sudden temperature change along the length of one of the hoses would indicate a blockage at that point. Also it is not possible to flush out the parallel-flow type condenser which is used in R134a systems. So the condenser needs to be replaced if there is a compressor failure or some other kind of debris trapped in it.
I have found that my '99 XJ is pretty sensitive to the amount of refrigerant installed in it. A little bit of overcharge and it makes groaning noises and spurts refrigerant out of the relief valve. The only way to be certain of how much is in there is to vacuum out the system and weigh the amount of refrigerant charged back in.