There are many differing views on what is the best disc or pad to use and it depends on how much money you're prepared to throw at it.
I don't have it anymore but I fitted Mtech drilled grooved discs all round and green stuff pads to my spider.
They gave a good feel and improved stopping over standard brakes for a very reasonable price.
One slight drawback they are a bit noisy compared to a standard disc when you stand on them.
You can also try changing the brake fluid for new and service the calipers and get the Pistons moving in and out nicely.
Thanks Ian m ,will have a look in the late spring,when i have more time.At the moment they seem to come to a gradual stop even though i had new pads fitted,when i bought her, in the late summer.They just seem poor for a incredibly fast car.
I heard that the massive more weight of that solution on the front axle is noticeable when it comes to acceleration.
I upgraded my 305mm last month with Brembo GT Sport, EBC redstuff and stainless steel brake pipes.
It helps, but you shouldn´t expect miracles. Not too good when the brakes are cold but after they´re warm, pretty nice and way more exact.
I had similar concerns for the first two years of ownership but managed to cure it last year. I had an issue where one of the pads was slightly askew in the caliper and stuck so not all of the pad was in contact with the disc. This also meant that after stops the hot pad was much less efficient giving a vague feeling and leaving material on the disc giving the 'warped' feeling. That said, it passed the MOT at least twice so couldn't have been affecting efficiency too much.
A caliper cleanup, fluid change plus some Performance Friction pads (everything else standard) by Dan at Autolusso Penrith made a night-and-day difference and my GTV now has brilliant brakes. If you can't make the ABS kick in at 50ish with a stamp on the pedal, I'd say you need a similar overhaul. If you've got the money a caliper refurb by someone like Bigg Red would be a nice treat. IMO, you don't need 330mm brakes, just properly working 305 with good pads.