Blow off valve or not?

Hello,

I'm running a K04 on my vRS currently running at 338bhp. The turbo is suffering from surging while under boost, which is related to the K&N induction kit not being big enough, so that will be sorted however it also surges when taking my foot of the accelerator and from what I have read this will reduce the life of the turbo.

So the question is, is it worth fitting a blow off valve or a better Diverter value? If so which one is best to go for?

Thanks