When my stock ko3s blew the seals there was no noticeable drop in performance. It blew the oil seals internally on the cold side so it did begin to fill the intercooler with oil.
Only way I'd noticed it was from a small rainbow under the car on a wet morning, traced it back to the boost pipework and then found a small puddle of oil in the pipes. Oops
I did get it sorted pretty sharpish though. If you're driving around for a while then it will lose performance from a loss of cooling effect in the SMICs as the driver's side one fills with oil, and also from the octane reducing effect of burning oil in the combustion cycle. (dilutes the petrol).