APT,

Nope they are not offering it in Stainless Steel it's aluminized offeset. 5x8x24 part number is 12289.

That's from Elis at Car Sound Magnaflow Tech Support

 

I haven't seen a #15609 in person, but every picture I see looks stainless to me. The wording is tricky on Magnaflow's site:

#15609 For F-150's: F-150, 4.6L, 5.4L, V8, Ext. Cab, Short Bed 3" Diam. Mandrel-Bent System, 4" Stainless Steel Tip, Side Exit

Some of the other systems say "Mandrel Bent Stainless Steel System", but not the 15609. Hmm, I just took a look at the article in Truckin magazine and it says:

"A close inspection of the MagnaFlow muffler revealed 16-gauge aluminized steel construction, featuring true mandrel bends and lap-jointed welds for tight seams."

Bummer. I have to get off the Magnaflow #15609 system push now. Might as well get the 3" muffler and their 3" tail pipe as I bet they are the same. I thought the whoel system was stainess. Stainless muffler and the pipe is $150 plus shipping and $50-75 for installation.

Okay, all new best F-150 exhaust setup:
Replace factory y-pipe with a Flowmaster for Magnaflow one (Mag shorter)
3" SISO muffler
3" Magnaflow tail pipe
Under $300 installed, $50 for a tip.