80km/h max speed on the current trains make airport service not feasible. You don't want to be riding in the LRT while vehicles whizz by on the QE2.
I'd disagree with this. As pointed out already, rail service that connects directly to the terminal would cut out parking, messing with the airport approach roads, and the potential for traffic snarls affecting travel time (I've had trips from the same place downtown to the airport take anywhere from 35 to 65 minutes) can make the train an attractive option. A car is definitely faster at times and from certain areas, but for a lot of the city, especially during rush hour, a car isn't as quick as we'd like.
My last big trip was to Chicago; I took the L everywhere there, including to and from O'hare. The Blue Line that connects the airport to downtown runs down the middle of a freeway, in the median. The trains top out at 55mph (89kmh), but obviously, with stopping etc they weren't going that fast much of the time. It was still very convenient to use, and with rush-hour traffic (which Chicago has pretty bad) the train was quite often keeping up or even passing cars.
There's also a lot to be said for the convenience of being able to get on to a single train, sit down, and have it take you where you want to go without thinking, navigating, or transferring. a long day of flying can be draining, and being able to relax and re-acclimate on your way home can be super helpful, even if it takes a few extra minutes.
Being able to take an LRT train all the way from the terminal and have it seamlessly turn into a regular train that cooperates with the rest of the ETS system strikes me as ideal. it would be a bit slower, as you say, because the trains would only be able to go 80kmh across the distance from the airport to the city, but once you make that journey you can stay on the train and it'll take you to any and all of the other stations, meaning transfers would be simple. I've used the 747 to Century Park - LRT home route before, it isn't that bad. the worst part is waiting for the infrequent bus at the terminal. i think that just by improving that frequency and getting rid of the transfer from bus to train would be a huge boon to airport users.