15,000 RPM was probably attained with a 10 x 4 prop. Thunder Tigers are good engines - I really like the Pro-46 version - but that is the maximum RPM achieved in the lab under non-field like conditions. Like the maximum highway mileage used by car manufacturers. Your mileage may vary . . . a lot. If you get 14,000 RPM with a 13 x 8 you're doing well. Isn't the 15 x 8 the normal prop for that engine, anyway?

Engines should run well on a test stand and similar to on the airframe. The exception would be if it was a cowled engine that was overheating - so unless you are using a cowl while on the stand that's not it. Engines do tend to run a little leaner in the air; hence the practice of richening them a few clicks from maximum while on the ground.

I don't believe switching nitro percentages will help you here. It would improve idle or ultimate RPM, but losing power after 10 seconds would indicate something else is at fault. How old is your fuel? Once opened it may only keep a few months (depending on climate & conditions).

25% oil is 5% more than most engines I am familiar with would recommend, and I don't know of a fuel supplier that offers it "over the counter". Are you mixing that up special?

If you get 10 seconds and then it drops off it could be several things. Overly rich, overly lean an air leak in the fuel line or a fuel delivery problem. If you had lots of vibration the tank would foam and that would kill the engine after a few seconds. The test stand would run better than in the model if that was the case. Did you balance that chopped prop? Does it just die or does it burble or rev up slightly before konking out? My guess would be towards overly rich because it runs but slower. Over lean would just stop, usually suddenly.

Pinch the fuel line and if it stops after eight or ten seconds with a little increase just before dying the mix is good. If it goes 20 seconds you are too rich. (If it dies in a few seconds without increasing you are lean.)