So I decided to run a couple of tests to see what kind of gain was to be had. I ran the tests on my own phone, a Motorola Droid 3.
I installed SD-Booster from Google Play. When starting up it displays the current cache size.
Next I installed SD Card Tester. This allows to do a benchmark of the read and write speed of the SD card. I didn't change the default cache size and ran SD Card Tester with 325MB file size twice.
The write performance seems to vary a bit between the two measurements, read seems to be around 7.5MB/sec.
Next I upped the cache size to 1024KB.
Then I reran two tests again.
With the 1024KB cache size it looks like both read and write is a bit higher than before.
Last test was to up the cache size to 2048KB and see what happend.
I only ran one test with the 2048KB cache size.
It looks like there is not much gain to be had by upping the cache size to 2048KB with this phone. But 1024KB looks like a nice improvement.