Who Has - I believe you, but it does sound as if you have a weird typo in your POH. 0-540 = 6 cylinders and 235 bhp minimum. If it's 160bhp it must be a four-banger. How many cylinders do you have?
LnS - When I started flying the Yak I too imagined that in "standard" UK weather compared to the frozen wastes, the operation of the cooling gills would be pretty much "open fully and locked". In fact I was surprised just how much scope there is for using them. In this autumnal weather, standard cruising will be at a part-closed position, and in anticipation of a prolonged climb I would open them , then if descending a couple of thousand feet I would pull them closed a little more. Even if not making huge power reductions it still cools down quite noticeably when you're swooping down for an overhead join.
Another advantage is that when you stop for fuel (something you get LOTS of practice at), you can close the shutters after shut-down, and the engine stays nice and warm while you refuel, grab a cuppa, etc, so the restart is easier and you don't have to wait around for ages again for it to warm up before departure. Oh yes, and leaving it in the hangar with the shutters closed can only discourage little nesting creatures, and those peculiar individuals at air shows who seem to find the oddest places to dispose of put their crisp packets, Mars Bar wrappers etc.
So there's lots to do with those odd-looking levers on the right sill of the cockpit, but we make up for the workload by having automatic mixture control, no tanks to change, etc.