Not True.
I needed 5 Achievements before TU2. then i started collecting the new achievements, i got 3 more achievements, and got 1000 gamerpoints, only because the achievements i got were worth alot more gamerpoints than the ones i originally needed, i havent got any more Halo 3 achievements since, and i have the Katana, and the security helmet. and i only have 47 achievements.
Alot of people have problems with this permutation in particular, there hasnt been a way around it yet.