ORIGINAL: 67Jag
Re temp and altitude losses: figure 2% for every 10F above a Standard Day and same for every 1000' above sea level. At your 60F, no losses; at 5400', perhaps 12%. So, your total loss w/ a good pipe, your engine, and that altitude, is more like 16%, or 3.5-pounds.
I suspect it's more than that, you have factored for the height but not that the ISA temp at that height is much lower than 60F. Therefore 60F at that height is hot. ISA temp at that height is about 40F, therefore you are 2 x 10F above ISA which according to your estimate means another 4% loss of thrust. Density altitude at that height and temp can be 8000ft or considerably more depending on pressure and humidity.
Harry