Trying to get more power out of an engine with 150000k's plus would not be a smart idea. It all depends on the increase you are looking for. It all starts as a couple PSI more on the boost and then generally snowballs.
If you have plans to rebuild the motor in the short term (6-12 months). Then why not see how much it can take? But if it has gotta last a lot longer, then keep it standard.
Well, thats my opinion. I am sure others will have different opinions.