I don't think you are reading the Jason specs properly...while they show the tubs with air and/or whirlpool, all (at least used to) are available without (i.e., as a soaker).
I find that I am much more comfortable with a nice slope at the rear which, as you've seen, shortens the 'floor' of the tub. This is one reason I opted to do the major rework to get a 6' tub (I'm about 6'2"). That 5' standard came about eons ago when the average person WAS shorter....just like kitchen counters at 36", designed for the average woman in 1950...people are taller now, and those standards are not a one size fits all. Bite the bullet and get a bigger tub, you'll be happier in the long run. BTW, I also raised all of my kitchen cabinets about 3", still not ideal for me, but better, and still marketable when the time comes.