The driver side sleeve is a little piece that fits inside the H-pipe on the driver side. It helps form a tighter seal because people had trouble with the nuts on the studs bottoming out before a good seal was made.
I just put the Mac o/r h-pipe in front of my Magnaflow catback and it sounds great. After I put the Magnaflows on, my first mod, I was kind of disappointed. I noticed the slight gain in power, but there wasn't much sound like my buddy's car and his Flowmasters. I picked up the Mac h-pipe for $130 shipped from Brenspeed. The sound alone is worth the money.
Install was easy, and I did it alone and with a pair of jack stands. The three piece design really helped. My car is just over a year old and had 30,100 miles on it. I sprayed the bolts with WD-40, waited about 2 hrs, then took the socket to them. They came off no problem. Everyone knows that you need a swivel on the passenger lower bolt and O2 sensor extensions for a 2004 model...with I have. One thing I did run into, I had to extend the left front wiring by about one inch. I screwed around with the damn pipe for a while, then I just spliced the wires. I added some home-made mil eliminators to the rear, and no CEL so far.
I did notice that both of the left O2 sensors were quite a bit dirty compared to the ones on the right. Any suggestions out there as to why? Thanks.