I don't think that keeping them in tins is the problem so much as making sure they are particularly good tins, such as the double-lid tins with a pry-out inner lid and an external slip-over type lid. Keeping them in the mylar bags inside the tins, however, does sound like a very good idea, as another layer of protection. It would also make it easier to reuse the tin for another tea without worrying about the odor from the previous. I will note, however, that sometimes when I've done that with samples, I've been able to notice the aroma of the tea coming out of the tin with the bag still closed, so I wouldn't trust the mylar bags alone.
Keep in mind, too, that tea in a container that is opened frequently will also go stale much faster than tea in a tin that remains closed. On the same note, a container that fits the amount of tea perfectly will keep the tea better than a container that also traps in a lot of extra air (ie, a tin that's only half filled). So, as Tea Nerd points out, you can always keep large amounts in a larger tin, with a smaller tin used for regular use to keep the bulk of the tea fresher by minimizing the number of times the larger one is opened.
Another thing to consider, though, is brewing parameters. There was another post on this forum about teas not being as good as the first brew. I was talking to a friend of mine about brewing, and he off-handedly joked that, in general, he always does things really well the first time, and then can never produce the same results after that. I think this especially applies to new teas. The first cup or two I end up paying a lot more attention than I do with subsequent brews when I just want a quick cup while I work. I have found, however, that later on, if I pay closer attention, I can usually brew a whole lot better, but in between I have a tendency to brew some mediocre cups.
I've had a few teas now where this kind of thing happened. My first batch of Lu An Gua Pian was great, every single cup. I thought it was extremely forgiving. The next batch I got of it (from another vendor), I found to be much more temperamental in brewing, but figured it was just that vendor's particular batch. When I got a third batch from the original vendor, however, I found it just the same as the second. It took about a week, but then I got right back into the habit of brewing it exactly how I like it. I've had the latest batch in tins for a few months now without any drop in quality. The differences in brewing parameters were very slight, but made a huge difference.
I also experienced the same with my Wuyi wulongs, which actually get better with age (so long as they are stored in an airtight container). Interestingly, they also would just not brew correctly when the weather got very hot here. Now that it's cooled down, they taste just fine again. I was beginning to really worry that at least one of them had gone bad due to how customs mistreated the package during shipping, but it's actually fine now. I've talked to others that noticed the same with the wuyis. Granted, this could well be due to a seasonal difference in local water treatment, but then again none of my other teas tasted any different.
Anyway, I've got lots of wulongs in tins (mostly Taiwan high-mountain and wuyis) and they are all doing just fine (they all have very tight fitting lids and an airtight design, though). I think the biggest thing is really just getting the brewing right for the particular tea. It takes some time to get to know a tea well enough to brew a consistently good cup.
One bit that I can add to the original question is that while Formosa really refers to pretty much anything from Taiwan, when you see something labeled simply "Formosa Oolong" or "Formosa Fancy Oolong", it's quite often referring to a Baihao (aka "oriental beauty"). I was just talking about this with a vendor, and it seems to be a leftover from old days. I think the Baihao was probably one of the first Formosas to find it's way west, but you probably shouldn't quote me on that