You are correct in that TRIM won't work on XP. This means that any deleted data won't actually be cleared from the drive. Garbage collection will help to alleviate the problem if given time, but unfortunately the C300 is very poor at this and takes too long to show any notable improvement. It's best used with TRIM. If you can't use TRIM for whatever reason, you should look at a drive with the Sandforce controller or an Intel drive as they have more aggressive GC. The Sandforce (not sure about the Intel) is also more resilient in the first place, and performance doesn't degrade as quickly as with the C300 when TRIM isn't available.
There is nothing inherently wrong with GC kicking in. There are some situations where TRIM alone will not improve performance but GC can. It's just a slower process, and does marginally increase the total amount of writes to the drive as data is shifted around.