So, with 10.4.3 now finally being available, I had to do a quick’n’dirty benchmark in Xbench before and after the update.
The test was run on a freshly booted system in a clean user profile with no background processes running. Four iterations for each OS version.
The test system was my rather slow 1.2 GHz 7455 Cube equipped with a Radeon 8500.

Overall 10.4.2: 34.99 (min) / 35.95 (max) / 35.65 (avg)
Overall 10.4.3: 37.37 (min) / 38.77 (max) / 38.08 (avg) ~7% gain

Here are three sub-scores that differed most:

Memory Test 10.4.2: 16.55 (min) / 17.31 (max) / 16.95 (avg)
Memory Test 10.4.3: 18.11 (min) / 18.52 (max) / 18.28 (avg) ~8% gain

Quartz Graphics Test 10.4.2: 45.47 (min) / 46.17 (max) / 45.85 (avg)
Quartz Graphics Test 10.4.3: 49.69 (min) / 51.67 (max) / 51.06 (avg) ~11% gain

User Interface Test 10.4.2: 30.52 (min) / 32.08 (max) / 31.25 (avg)
User Interface Test 10.4.3: 34.92 (min) / 36.63 (max) / 35.66 (avg) ~14% gain