Yahoo! has enhanced the APC opcode cache to support PHP-5.1. 

PHP-4.3.11 (optimized)       176.135 fetches/sec
PHP-5.1 SWITCH               195.344 fetches/sec
PHP-5.1 GOTO                 198.707 fetches/sec
PHP-5.1 CALL                 199.342 fetches/sec
PHP-4.3.11 (optimized) APC   209.342 fetches/sec
PHP-5.1 CALL APC             252.968 fetches/sec
The different VM models in PHP5.1 are very compiler sensitive. The following tests were run on a FreeBSD-4.3 system and compiled with gcc-2.95. On more modern compilers you should theoretically see better numbers from the GOTO VM, but the differences are usually not all that dramatic.

Try it
~/php51/Zend> php zend_vm_gen.php --with-vm-kind=GOTO
zend_vm_opcodes.h generated succesfully.
zend_vm_execute.h generated succesfully.
cd ..
./buildconf
./configure ...
make