Sorry but you're wrong
The 30kB bug has been solved with Optiboot 4.4. It was the issue #380
http://arduino.cc/forum/index.php/topic,64105.0.html
Google Code Archive - Long-term storage for Google Code Project Hosting.
Sorry for the misunderstanding. I meant that I have only tested 4.5 on the 1284p, not that the bug is on the new version 
Besides, I am up with 4.5 with a 16MHz resonator and when I get a chance I will try the cap/res filter technique again with external crystal.