This doesn't answer your question, but might be useful information if you are starting to shop for a tub.
When you are in water you are something like 90% bouyant. Your full body weight will not hit your pressure points the same way, which is why on many tubs you see you may not see the same sculpted features as in, say, car seats.
This is one reason why wet testing is imperative over "dry" testing a tub: you will weigh *much* less in water, and the water acts to lubricate making you slip around on the seat a little more (especially on the cool looking pearl-like finishes) .
Where lumbar would go, many tubs have jets. Jets turned on will give you a gentle push (all the way up to a not-so-gentle push depending on the tub and the jet setting) which may give you just as nice a feeling. In truth, a lumbar ridge may be less comfortable as a "slipping around" issue.
Again, totally doesn't answer your question, but just wanted to toss up an idea or two. Don't throw out ANY tub in the running until you have wet tested it.
-Ed
