My slightly modified '13 (SLP pipe and head) is the same too. And I noticed it while still stock but it ran about 5 or 10 secs longer to each point
10 secs good , 15secs starting to notice it, 20 secs and it holds steady for power, just less.
The thing that is consistant here for me is the coolant temps at these points. 125 all good, 129 starting to fall off, 131 to 133 steady temps just less power.
It's not the tunnel cooling system either cause it happens with waist deep and just 6" fresh the same. And,, as soon as you turn out or shut off within a sec or two the temps fall to 123 125 as quick as you can get your eyes back on it.
It seems to me that either the sensor location or the engines internal coolant flow (MO) is lacking some what or something.
Obviously the less power you are making or asking from the engine through clutching the less loss you would notice. I actually lightened up on the load I was asking from the engine with clutching to balance this out.
I just know there is more there.
Anybody else putting this power loss in line with coolant temps?