You fell into the aeroplane salesman IAS/TAS trap there
At FL100, 138kt IAS is about 155kt TAS. So you are 9.7GPH, I am 10.5 GPH. That is an 8% difference; probably accounted for by the difference in the cockpit frontal cross-section alone. You pays your money and you takes your choice, and width matters more than length in this case
I wonder why your LOP figures are so much slower. The fuel flow doesn't have to be that much lower. There is no efficiency gain in going that far LOP. The tiniest amount past peak EGT is every bit as good. Perhaps you cruise at 75% or higher power.
The thing that you pay for heavily is flying at max cruise, versus flying just a bit under. I get the impression that a lot of Americans fly at 75%, and well rich of peak. Hence the old saying that fuel is the cheapest thing you can stick inside your engine. Less and less true every day, but it undermines the decades-old preconceptions about some planes being more efficient. It also makes comparisons of different planes' cruise performance, from published data, nearly impossible.
Pianorak - there are grass strips and there are grass strips. Some are perfectly OK, e.g. Panshanger. Many are dreadful, with potholes which the owner would not tolerate in his back garden, but if somebody bends their plane on his airfield he sticks his finger up and tells them to claim off
their insurance. My view is that grass is OK for any of the TB series, but it's true for all planes that operating from grass will result in higher operating costs in the long run, as well as a generally filthier plane all round.
A TB20 with a 3B prop has 8" of ground clearance on the prop. The front gear suspension travel is 3" (probably same for all TBs, old and new) which leaves 5" max allowable pothole depth. However, this 5" can be exhausted if the ground ahead is rapidly rising, as often is the case on grass-concrete transitions. One must have a walk-around before departure, always, to make sure one can taxi away, and use the towbar if necessary. Even if this p1sses off a queue of renters behind.