My mechanic has just told me that my front right hand strut is leaking oil.

Any advice from anyone who has replaced their struts with high performance ones like Koni?

I will be replacing them on Monday but any suggestions would be appreciated. I might replace them all in one go.

By the way my car is a TH magna sports lowered aswell

Yes it is a pain installing KONI *inserts* which are not full struts. You need to do a cut n shut on the stock struts and bolt the KONIs into the butchered stock strut housings. You are looking at an extra hour of labor for each shock to do this, over fitting a replacement full strut shock. The rear is pain free though. Depending on how neatly the job is done you may need a camber kit to correct the quarter or half a degree the alignment might be out by afterwards.