One possible catch is rear derailleur cage length. If you have a triple crank, the long-cage derailleur is advisable. The Performance upgrade kit doesn't specify which cage length is included, so it's probably the short-cage version.

Personally, I prefer Sachs/SRAM chains over Shimano, too, on the basis of strength. The Sachs PowerLink is a nice touch too. This might be another reason to buy the parts separately.

As for what cassette to run, if your T1000 is a 2002 bike then it comes with an 11-28T cassette. The 11-tooth may be overkill since you have a 52T chainring up front. If you do want to keep the 11T for a killer high gear, then Shimano has an 11-21 or an 11-23 to offer you in 9sp. Or you can think outside the box and go with a mountain 9sp cassette such as a 11-32 or 11-34 and use a DeoreXT rear derailleur.

If a 12T would be a high-enough gear for your needs, then Shimano has 12-21, 12-23, 12-25 and 12-27, or 12-34T.

Basically, the cassette choice should suit your needs, so think about what gears you use and what gears you don't use.