We just tried a pair of 28 T-Servs on our tandem- as you said very grippy but we lost about 2-3km per hour compared to our normal tyres and really lost it on the downhills. BTW these were new formulation 2009 T-Servs.
If you want to stay with a 28 try Conti Gatorskins- we have had great success with these (same weight as the T-Servs but faster) and very puncture resistant (we don't wear out tyres- they get sliced up by glass in NZ). The T-servs only lasted 5 rides, we get 3-6 months out of a Gatorskin.
I ride Gatorskin 23s on my single as well.
If you like Panaracer tyres I noticed on their website a brand new tyre for 2009, the RibMo that comes in 23, 25 and 28. This is a better tyre again with less tread so should roll more and even more puncture resistant. Both this and the T-Serv are commuting tyres BTW.