I must admit that when I first started running the big bore shocks they kept being removed to compare with the small bore ones.
The issue with the big bores is lack of setup development time for tracks in this country.
There are a few of us who have puts some serious hours in working out what pistons are required (IMHO the standard pistons aren't suitable for the rear end on our tracks ... I drill to 5 x 1.4 at the moment) and al of us, that I know of, are now in agreement that once you drill the pistons which allows you to move in to the same oil ranges as you would of on the standard velvet shocks the big bores ARE better.
There is a slight issue with front spring selection, basically the combination of our surfaces, track layout styles and tires mean we benefit from a stiffer front spring that exists in the Kyosho / X-gear range. Losi Green/Blue and Yokomo Pink/Green (similar real rates actually) seem to be coming out as favourite.
Start off with a Kyosho / X-gear Red rear spring and your 95% of the way there
All that said if you can get the ZX5-FS2 for around the £300 mark then that represents fantastic value!