If your hitting the S300's well, then I'd probally go with what Lowpost said and look at going with the 6.0's. The 6.0 and S300 are for the same SS at 80 - 90 for a 5 Iron.

Like I said my normal SS with a driver was at 95 and my 5 iron was at 82. My shaft for the driver is a stiff and the iron shaft's are a reg+ ( just slightly above reg )

If you want to be sure for you then go get your SS checked with a 5 iron at a normal swing not swinging out of your shoe's and test them because all shaft's are not equal.