The manufacturer will tell you that a cap tube system is critically charged , they are to some extent but you can cut off up to 200mm on some and be confident that it will still run to design conditions.
It's true you can cut off a significant length of cap tube and still have good performance, but the key to this is the system charge.
If you have a system which originally required 16 oz of refrigerant and you shorten the cap tube, it won't perform quite the same if you recharge with that same 16 oz. of refrigerant. A slight adjustment in charge will usually bring it in line.
For a given system, a shortened cap tube would require a lower amount of liquid subcooling entering the tube...ie, a slightly lower charge.
Of course, there would a limit as to how much you can snip off.
You know Pascone it sounds like to me your system is contaminated. Sporlan has come out with a new cap tube drier that is suppose to remove particles as small as like 5 micron. I tried one on a contaminated sytem like yours and the cap tube still clogged up in less than 1 year.
All you can do is uninstall the compressor and dump the oil and repalce it till the cap tubes stop clogging. The particles will be in the oil and in all the piping.
Tell your customer to buy a new unit or just replace everything! Evap coil piping and condenser unit. Its really all you can do.