I've owned the HD595 and it's the best gaming headphone you can buy second hand for the money apart from the AD700.
HD595 is worth like £50-£70 and the Q701 in the UK is like £220+.
I doubt it's worth upgrading tbh, there isn't £150 worth of difference for gaming. If you could get the Q701 for the drop price of £100-£120 then yeah it would be worth upgrading no doubt.
HD595 is better than a stock HD558 for gaming (i've owned both).
I'll list what I own;
HD600, Fidelio X1, Q701, HD558, HD25-1 II
Owned in past;
HD595 and HTF600
I sold my HD595 for the HD558 because I am going to mod the 558 into 598's which are basically an upgraded 595, sold the 595 for like £50 and paid £80 for a brand new pair of 558's, that was worth the £30 upgrade after I mod them when I get some time to do so.
If you own the HD595 there is no point upgrading from them from a gaming point of view (competitive gaming that is), unless you can get something comparable to Q701's for cheap, for music and movies though I would upgrade to something better if your using them just for gaming I'd wait for another deal to come along.
In fact the SoundMAGIC HP200 Headphones are on sale atm on massdrop, go for them if you want a cheap upgrade, they retail for £220 here in the UK, and massdrop have them for cheap. Use the link below to sign up
https://www.massdrop.com/r/6HW75F
then search for them, only 2 days left so be quick they are superb and worth upgrading to for gaming, music and movies, they are also low impedance so don't need an expensive amp.