The effect on piston-engined aircraft would certainly be reduced, in part due to their lower speed, though the biggest issue is altitude. As per Eric Moody's experience, volcanic ash tends to be pretty high up until it dissipates. Capt Moody's 747 suffered a 4-engine flame out twice, but in the lower air they managed, twice, to restart the engines.
Were a piston-powered aircraft to encounter that level of volcanic ash, my opinion is that it would eventually clog up the air intakes and some would enter the engine causing severe wear on pistons, valves, etc. I would imagine there would be a loss of power before engine failure though, which a good pilot would take as a warning to get on the ground asap. There would still be wear on the windscreen (though not as bad as at jet speeds), and likely wear on the skin of the aircraft. Any fabric-covered airframe could be in real trouble!