You seem to have found out the limitations of not enough air, you can keep pouring the fuel to it, but that does only so much. Your Sig list a 4" exhaust, but you said you don't have one?? Anyhow, the steps you need to follow will address these issues. You seem to have most of the go fast goody's just not the basics. Open up the air way and let her breath, your truck is starving for more air. Pick anyone of the popular intake air kits, I have the AFE on and like it very much. Next I would consider switching to a larger exhaust housing, you tow alot so the 16 cm/sq one would really move the air and lower the EGT's. It is a relatively cheap up-grade, if you have the funds you might even take it to the next step and get yourself a better turbocharger, you make the decision. Lastly, if you do not have the larger exhaust, now is the time to get one. This is a very beneficial up-grade, to move more air you need a bigger pipe. But this does not mean the truck has to run a straight pipe or be loud, most all systems include decent high flow mufflers that help calm down the noise. Some times Drone becomes an issue, I had this problem and solved it by add a Aero Turbine muffler in front of the system muffler, a bit pricey but worth every penny.
This is a great list of modifications that will wake your truck up and help make full use of the mods you have already made. You can control the cost by selecting the parts that fit your budget, but remember nothing on these trucks is cheap and quality does cost more. Good luck