^yeah what seanbeckaroo said is right, when I towed the 4000lbs with my truck I was on the 87 performance tune, using 89 octane. It's a PITA to switch to the tow tune every time so just fill up with higher octane. Seems to get better mpg anyway from my experience. I will say the tow tune did pull a little better at the lower rpms.
It's pretty common for people to tow on performance tunes with higher octane fuel. Check out the superchips forums, it's actually usually preferred over the tow tune...
And not only that, but the tow tune requires 91 octane. I was able to increase capability over stock, and use 89 octane(cheaper than 91).
Breathing in and out is good enough for me.