there are many tried and tested tuning routes for the RS turbo. unfortunately there isnt really an accurate definition of "SAFELY" as you'll probbaly be talking about a 15 year old car with 100k miles and unknown hstory, you never know what could happen.

generally speaking most people tend to start with the basics: exhaust, filter

then youll want it chipped (which just removed the boost limit)

then run some healthy boost through it (which WILL shorten the life of the turbo if standard)

you'll probaly be knocking on 170-180bhp

deffo take it to a tuners with experience of RS Turbos tho, as youll probably find its not set up properly and running less than standard power anyway, i recon about 80% of them are