Ben--
I use a Teeny 350 on a Sage SP+ 9-foot 8-weight.
Pros: the Teeny 350 sinks like a rock, and shoots like a rocket. The 450 must be downright nuclear.
Cons: (somewhat mitigated by the fact that it seems you are using this as an extra spool, whereas I use it as my primary line). As evidenced by my crustacean near-grand slams, a shooting head, super-fast sink line is not always the most desirable from shore.
Also, Gene Bourke at The Sporting Life Orvis store in Mashpee last week telepathically described a problem I've had with my Teeny line. I went into the store looking for some 303 line protectant, as the running line had been getting almost a gummy feel and had been tangling a lot. I am religious about cleaning my fly line. Gene said that some fly line cleaners have detergents that can also strip the finish from a line. Point being, he nailed it on the head when he asked if my Teeny running line was kind of globbing together. Couldn't believe it! He also recommended I get a can of spray silicone at a hardware store, spray it on a rag, and use that. Hell of a lot cheaper. And great for business, too: I won't forget that Gene was truly interested in solving my problem, whereas he could have sold me the Orvis product there in the store. Thanks again, Gene, if you're reading this.
I say all that to say that I've had problems with the running line tangling with my Teeny, although this perhaps has been ironically caused by OVER cleaning it--ask anyone I've fished with; they've seen my tangles. (And not only do I clean my line, but I also stretch it, although not as often). As I mentioned, Gene apparently has also noticed the gumming-up quality of the Teeny running line. Next time around, I'm going to try Orvis' Depth Charge; perhaps its running line is harder. You might consider that as well.
--Ric.