to be fair, he did not say that you will not see any improvement. he just said that well, dont expect much. it is true that your cpu is a bit on the low side nowadays.
and for future reference, such a question is more useful BEFORE you buy your upgrade. asking it now will just give you a heads up, of a few days maybe. but since you already bought it, it is what it is unless you do something.
but coming from a 9400, you should see decent improvement, just dont expect to max out heavy games.