Hi,
I have just had my air-con fixed. The cause of my loss was leaking condensors - a common fault on Boxsters. So I suspect that a re-gas will not cure the problem. The usual thing is to put gas through the system and use equipment to detect where it is coming out - before just re-gassing. However by looking through the air ducts at the front, you may well see damp patches in the bottom corner of the condensor(s) - that is the give-away that they are leaking and need replacing.
Mine lasted 11 years (but only 58K miles), which is exceptional. The air con specialist I took mine to had done about 30 Boxsters this year and kept a stock of the condensors!
If you live in the East Anglia area I would recommend ACautoworks on 01473 313131 (or look up their web site http://www.acautoworks.com) - I was pleased with what they did for me.
Regards,