Big Pistons - I also fly geared engines, but the GTSIO are especially highly strung. I always quiz my old Commander mechanic (he knows serial numbers by heart after 30 years of only doing Commanders) if the old GTSIO-powered 685 would be something I should step up to at some point, fully knowing it's his least favourite of them all. Always gets him going. He says they rarely make TBO and often have to have some top work done before hitting 1000hrs. Sluggish take-off as well, even though they have all that power. They do perform like turbines up high, though.
I think the part of the recommendation in the manual you talk about is probably due to gearbox limitations rather than a cooling thing, no? The gearbox chatters and doesn't like to be pushed by air. I always try to fly mine with positive drive and have been cautioned to never pull to idle ever, except in the flare. It sometimes means she's little harder to slow down, but if you dump gear early, it's normally not a problem.