The brakes on my old 3.9 Vogue SE were the best I've ever experienced on a car, let alone one weighing over 2 tonnes, so a standard system that is working well is up to the job.
You can get front discs and pads for £40 and the rears for £35 from Paddocks, that is as cheap as you're ever gonna get, so I'd find it hard to justify paying nearly 5 times as much for grooved discs. Mind you, haven't had these Paddock ones I've bought for long so can't vouch for their longevity yet.