I do not know the Katy Trail specifically, but I ride alot on hard crushed limestone trails around Minneapolis often riding 25-40 miles. I would strongly suggest running as big of tires as your bike can take, especially if you are a bigger rider or carrying gear. I am 225 lbs and usually ride 25-28c on the road and use my cross bike on these trails with 35c at around 75psi rear and 70 front. Makes a huge improvement in comfort and speed. Also, if the weather is dry, crushed limestone gets very dusty so you may want to run a dry vs wet lube. The limestone dust is very abrasive so I would also recommend wiping down and lubing drivetrain each day. I like boeshield T-9 or White Lightning clean ride lubes. As for speed, I notice about a 1-1.5 mph decrease in speed for a given level of wattage. Although not terrible, rolling resistance is higher than pavement and coasting does not get you very far before you notice a drop in speed and need to pedal. I always feel as though it is just slightly harder to pedal than pavement, enough that you know it is there but not like tying to pedal through mud, more like running tires really low on air, just sluggish vs road.