I am assuming that the values you mention are on a range from 0 to 100%? If so, that indeed is a bit steep, my curves are usually a lot flatter than that.

My mechanical set-up aims for a linear and symmetric mechanical link from servo to carb. Meaning: servo arm length as identical as possible to the carb lever, and the levers "parallel and symmetrical". As in both being at a square angle to the linkage rod at the mid of the stroke.

I checked a few of my throttle curves, and they vary a bit, but the .91 fourstroke has as last three points (from -100 to +100) +24, +56 and +100
The .91 twostroke has +22, + 55, +100 as last three points
The .52 fourstroke has +19, +55, +100 as last three points
The 65 cc radial has +8, +49, +100 as last three points.

The other planes are similar as they have basically the same engines.

On average, my throttle curve seems to be (9 points, from -100 to +100) -100, -83, -67, -50, -30, -5, +24, +56, +100 as curve.

YMMV of course. I have had no issues with linear throttle curves with respect to getting the engines adjusted, but I did have issues with flying like that. I don't like a too agressive throttle with the stick all the way back, makes landing harder (my flying style basically is to pull the plane in on the engine, so I like a bit finer throttle control near the lower end.

I cannot comment on the tanks. For some reason, all tanks I have used so far all turned out to be gas resistant and not softening up over time. Usually I use what is in the plane when I get it, if I need to buy a tank however, it always is a Dubro. For some reason I really like those "chinned" tanks.