Dave,
there are various different ways to attack this one. And a few people have a similar problem.
At the moment we have the viscous plus an electric fan, but have also inserted some baffles into the nose cone to ensure that all the air rushing in, is directed at the rad' and doesnt go anywhere its not needed. There's also the possibility of adding some cowling between the viscous and the rad (we played around with some off a B reg Carlton to ensure that it can draw hot air away as effeciently as poss. both of these though are just trying to get the max performance out of the fans you've got. If not, someone used a fan from a diesal something or other that sounded like it was a monster performance wise, but i cant remember who that was, micro are good choices i think, and i think firm called "Kenlowe" (correct me if i'm wrong) are decent makers? However, lots of people dont seem to have any trouble at all. the viscous one does a good job for us, and the electric didnt kick in at all on a recent 200 mile round trip. If the fans just cant cope you could always add more venting, quite a populkar one is inserting to large side vents, which i think big jim amongst others has, but they can be got from most marine chandlers. This might get you more of your hot air out of the engine.
i'm sure others will have more detailed infor for you. (waiting to be corrected!
cheers
paul
by the way, someone at a well known and now defunct garage decided that he'd do away with our viscous fan as it was "rubbish" and replace it with an electric one as yuo were thinking about doing. Two miles outside the garage the engine "decided to make a cup of tea" and we were calling the AA. Viscous now back on, electric now positioned and wired the correct way around (as it was designed to according to the manufacturers leaflet iside the car - doh!) but not really being used except in extreme situations