SS, whether it's 304 or 321, is heavy as fawk.

Burnsstainless.com sells stock so you could make your own Ti downpipe after you find your axle-back. It's not that bad. It's like $33 a foot. You'll need about 8-9 feet if you're following where the factory mid pipe runs, and that would cost less than my Apexi 3" downpipe did new. You just need to find someone with the equipment to weld it.

The worst part about titanium is finding someone that sells big enough stock to be used for an exhaust. 2" is the largest I can find anywhere. You may also want to consider Inconel. It's got great heat resistance properties, and it's also very lightweight. Doesn't really cost any more than Titanium. But it's also very hard to find in diameters larger than 2"