Eagle2000GT

Interesting dyno charts. Extrodinary gains especially on a n/a motor. I put on exactly the same LTs and mid-pipe on a supercharged motor and saw no gain at all on the dyno. Like you the dyno runs were months apart but the tempature variation wasn't as extreme as yours. May 30th and Jan 17th. The cool tempatures in January probably helped. Are these adjusted or unadjusted numbers?

01GT4.6

no offense, so don't take it like that but I don't believe those numbers one bit. There is no way in hell you saw those actual gains with only those mods. The a/f looks alot better on the second run which is probably why you saw a gain and dynos are never that accurate (hell I've seen my dyno numbers change 10hp with no changes to the car or tune, only dyno & day). If you had before and after track results where you dropped .2-.3 sec and gained a couple mph, I may believe it.
Out of curiosity, did they dyno shop, recommend or sell that setup to you?

Purostaff

These are SAE corrected.

Eagle2000GT

Most people do not see those types of gains. The new tune with a leaner A/F mixture obviously helped. My first tune was 385/370 on a Mustang dyno but I was pegging the MAF and the tune was very rich mid-range and lean at the top. A few months later I had BBK LTs and catted x-pipe installed. The first pull before the new tune was 385/368 on a Dynojet. Almost exactly the same. The headers and mid-pipe showed no increase at all. After the MAFia was installed with a new tune fixing the lean A/F mixture the Dynojet said 370/377. I dropped 15 peak rwhp because I wasn't running as lean but I picked up 15-20 rwhp mid-range by not running as rich. These pulls were all on the same day.

The new leaner tune and a cool day probably gave you as much or more or an increase as the new headers.