Yup, two to three years is alot to ask of an impeller. And for the little bit extra it costs I would get the OEM replacement. Some of the aftermarket impellers have had issues.

If you have an 89, there are no separate risers to check, those are the one piece 'batwing' manifolds that OMC used from 86-90 or so. You can take em off, and look at the exhaust opening, there are 4 openings that the water exits from, make sure they are nice and open and there is no flaking rust. If they are plugged with rust you have a problem, because they are not available anymore, unless you find a set on ebay. There is a conversion using Ocso manifolds and risers being sold by one of the marine retailers that will work if you need it but it's pricey. I have em on mine, put a new set on in 2004 and have a spare set for my next change in few years. If I still have he boat at the next change interval (here in salt 4-6 years) I'll go with the Osco conversion.

Other point, DO change the impeller (very easy on a Cobra) BUT make sure that the gasket on that impeller housing seals well, if it doesn't, when the boat is on plane, the upper gear housing is out of the water and the impeller housing can suck in air and lower the water output. When you check it at idle, the upper housing (where the impeller lives) is under water so you would not notice this. It does match your symptom of OK water flow and normal temps at idle and low speed, and overheating when the boat is on plane.