I ran 23mm - 25mm tires on T217 rims for about 4,500 miles so I don't believe your 25mm target width is a constraint. However, I've provided a link to a compatibility table lower on this reply which might be of interest.
At 330lbs, a 25mm is also OK for a performance / fast recreational tire, but 28mm is still a better choice for all-around use.
As for your tire choices, compound durability is really variable depending on what your roads are like. My general recommendation for folks looking to change tires is to check with other tandem teams who ride in your region to find out what they have had the best luck with. We have friends who live in Bend that started a YahooGroups forum called "BOBCATS" http://sports.groups.yahoo.com/group/or_bobcats/
Their forum might be a good place to ask if you don't have any regular tandem riding companions in your area. Eugene is another hotbed for tandems.
My experience with Michelin's Pro Race tires is great performance, short life... particularly on a tandem and even with our silky-smooth roads here in the Southeast. As for the Gatorskins, they would definitely feel quite different (harsh) compared to your FasGrips, particularly if you're running the newer Carbon 12's which have a very supple sidewall (almost too thin in some respects). However, the Gatorskins are certainly quite durable; no question about that: just not sporty in their feel. Notwithstanding the thin sidewalls, the FasGrips are still one of the better "performance tires" for tandems and the 28's are a really nice size for most average size tandem teams.
If you haven't read it, Mark Johnson of Precision Tandems in Kansas has put up a pretty good Web site that discusses many of the issues associated with selecting the right tire for a tandem. http://www.precisiontandems.com/arttiresbymark.htm
As for matching tire width's to rims, Sheldon Brown has a nice table on the Web. Please note, you'll need to measure the inside width of your T217 to use the table.
Hope this helps.
Note: We have been running 700x23 & 700x25 Vredestein Fortezza's at 145psi on our road tandems since 1997. A very good performance tire with moderate life. However, not ideal if you have to encounter too many road hazards as a casing cut usually does-in the tire -- even beyond salvation by Superglue.