All good bikes. I'm sorta partial to Specialized, mostly because my local shop is good and I haven't had any problems, but for what it's worth we just got a Sirrus for my son and it seems to be suiting him well. Ride and choose.
However there is another thing you might want to consider. Try some narrower tires that will fit your rims, as smooth as you can find. That will make your pedalling effort easier for a given speed. Possibly change the cassette to some taller gears. A lot of what hurts performance would be those suspension forks, so if there's a way to adjust them and make them stiffer or lock them you might like the bike a lot better. You might find that you can make some inexpensive adjustments and get what you want out of it.
However...The Sirrus and probably the others you are looking at are probably a lot lighter and stiffer, and should have slightly taller gearing, as well as narrower tires. The Sirrus 28 mm tires are half as wide as the 1.95" on the SC30 (which isn't really a mountain bike, it's a comfort bike) so it will perform a lot better. One other note, if a road bike is properly fitted to you and set up right, it should be comfortable and should not hurt your back. Some light core strength training might help, but I have minor lower back pain often, and doing 50 miles on my road bike doesn't bother it a bit.
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Tom
"It hurts so good..."