A variety of opinions - thanks all.
To be clear, I'm not enquiring because I think changing pedals will make me faster. I've been road cycling for about 10 years. I am already as fast as I will ever get (or can be bothered to get) now!
I'd heard that SPD-SL's and the like could be more comfortable because of a larger contact area, resulting in fewer "pressure points". On long rides I get numbness in one of my toes, but perhaps this is just the way it is, and unlikely to be solved by a different pedal system.
I wouldn't consider going back to flat pedals. I slipped off in the wet a couple of times in my early road cycling days, pushing away from a set of traffic lights. And since the second time I nearly went under a skip lorry, I'm off them for life (except to ride my "pub" bike, which never goes any further than... well... the pub).
So reading through this thread it seems the general consensus is that a more road focused system is unlikely to be different enough to justify the outlay. Thanks all for your thoughts though. Interesting to hear different people's points of view, especially those that have tried both systems