That actually sounds about right for that cooler..Maybe a little high but not too high..
I never heard of that cooler, but did a google lookup. Seems like all the sites that have it are *.de's so doesnt look like its availble in the US market.
http://www.easy-mod.de/default.php?seite=arctic_pro_2
On a Tbird 1400, witha case temp of 22 degrees, thier CPU is at 46 degrees and at full load its at 55 degrees. Thats 24 over what I assume is ambient for idle, and 33 over case for full load. With thier case temps this low, I assume that thier case has very good cooling and that case side panels were probably off, or the board was not in the case at all, as most places do with heatsink testing. Now the Athlon 2000+ puts out slightly less heat than the TBird 1400, but they are comparable in ther watts of heat output.
Your numbers are 36 over ambient and 29 over case for idle, and 46 over ambient and 39 over case for full load, which I think can be due to just having it in a case. Even though this Heatsink is certified for cooling up to an Athlon XP 2600+, I dont think it does the job of cooling anything over 1.1Ghz or so sufficiently, which is low 50's for these higher end XP's in my mind. You can do all the heatsink tweaking you want, and maybe drop it down a few degrees, but you probably wont fine much better results outside of getting a new heatsink unit.