I don't think you can make a general assumption on that. It depends on the number and length of the SQL statements you have.
String handling is one of the more time consuming things while monitoring so it could cause some overhead if you have a lot of very long statements, but it's hard to quantify that.
Josep, There is no hard value that can be identified to this type of change. The overhead increase depends on a variety of factors, such as:
1) What value of SQL String capture is defined
2) How long are the SQL strings that are captured.
3) How often the SQL strings are executed
4) What is the response time of the Purepath. A fixed increase in overhead still creates a percentage that is variable based on the total PP execution time.
Best way to measure is simply test it with/without these changes.
I would guess you'll see a value somewhere between 0%-30%. Sorry I know that is not very helpful.