Hi there, I'm playing on as Custom Modpack and Fastcraft yes, worked for me once, and then never worked again, I tried the Debug Versions and Version 1.7 and 1.6, 1.7 Crashed without even loading the startup screen and 1.6 Starts fine but I see no FPS Increase unlike my first attempt at 1.8 (Which was a 30 Frame Increase, while 1.6 was my regular 5-10 Poopy FPS). Here's my crashlog, hope you can fix it.
Crash: http://pastebin.com/xjcmDzng
Also the crash report is showing an MP Server I was testing it out there but the same happens in SP so no difference.