19.5s are true medium-duty truck tires - like what comes stock on the Ford F-450/550/650 size trucks and some semi-trucks.
One would expect 19.5s to get much longer treadlife and mileage and higher load capacity than conventional LT tires. On-road traction should be equivalent, but it will be harder to find high-traction, off-road, 'mudder' type tread in the 19.5 size. 19.5s will be more expensive, and one probably won't find them at your corner NTB/Goodyear/Firestone tire shop - only at heavy truck tire shops.
The sidewalls of 19.5s will be MUCH stiffer & shorter than in conventional LT tires, which will contribute to a much firmer/stiffer ride. I'm not sure about speed rating differences, but I can't image 19.5 tires would have dramatically lower speed ratings. If you're hauling a +10,000 pound load with your race car, tools, trailer, etc... you're asking for an accident going much over 80mph anyway, due to the stopping distance required & the idiots that play in traffic these days.
I've considered going with 19.5s on my 3/4 ton Dodge/Cummins Ram, but haven't had the $$ to do this yet. I'm thoroughly dissatisfied with the ride of my 265/75/r16 Bridgestone Dueler Revo tires, because I feel the tire sidewall 'squirm' under cornering. They feel really floaty and disconnected - kinda like riding in an old-school Caddy, Buick, or Lincoln. I don't like that one bit. The Duelers have much better wet, mud, and snow traction than the Michellin LTX M/S that they replaced.... but the LTX ride was firmer and not 'squirmy'.
My Ram felt like it had near sporty-car handling with the LTXs. The Revos completely changed the ride characteristics for the worse. Both LTXs and Revos were E load rated. Go figure...
Rickson Wheel Manufacturing (http://www.ricksontruck.com/)
has a lot of information for making the change to 19.5s. They claim tire mileage life increases between 2-4 times, which helps justify the higher tire prices.