I'm not sure what this thread is predicated on. where/why is the T3's MP count different from the 450D's?
T3 - 4272 x 2848
XSi - 4272 x 2848
yes they may have updated the manufacturing process (I don't know enough about it to know whether changing to gapless microlenses is a big deal), but I also don't know where the assumption that the sensor has changed is coming from. given the fact that the max pixel counts are exactly identical, and that the T3 slots in even cheaper than the XSi, I am of the opinion that it's exactly the same sensor
The radically different high ISO performance for one. Even with PC powered NR in post production the 450D isn't *close* to that good at high-iso.
Updating from Digic 3 to Digic 4 likely adds at least 3/4 stop. Better noise reduction algorithms probably adds 1/2 stop, Changing to gapless micro lenses and updating the bayer filters, can add 1/2 stop, suddenly you get around 2 stops with the same basic sensor. They can also make a lighter AA filter. All this could make a big difference.
The simplest solution is usually the best one.