Your x264 command looks a little wonky.
You specify 2 pass mode with a bitrate of 15mbps. Then you set maxrate to 2.5mbps. This doesn't make sense
Did you miss a 0 in there? Also, you should specify --vbv-bufsize. 25000 should work for both, but I'd check the BluRay compliance threads to make sure you're doing things properly.
Also, in you avisynth script, you should add interlaced=true to ConvertToYV12. As-is, you're assuming progressive content, which will bork up your chroma