Dump the secondary cats, they are worthless! Nothing is monitored on them (except in the older versions of the Box - you know who you are). The exhaust is stifled at that point too. So not only are you dumping weight, getting a much nicer exhaust note (barely noticeable but there), you are also doing it for virtually no cost! What's not to like? But...no pressure! Top Speed Pro1 makes good cat bypass pipes. If you sell the secondaries on fleabay you can recover most all the cost (about $150).
From what I have observed you may want to consider replacing the Thermostat with a low temp version PRIOR to installing an UD Pulley. Simply put, the devices on your system are running slower b/c of the UD Pulley. I noticed a SLIGHT INCREASE in engine temps as a result. It wasn't much of an increase...but it was not that hot yesterday and I was not running the A/C. With the addition of a low temp thermostat you can be sure your temps will not exceed specs IMHO. But don't let me pressure into that upgrade either.
When you are talking getting rid of the PRIMARY CATS and Headers...that is whole nother kettle of fish! (mine's running great BTW)
...no back pressure!