Additional cores only increase speed when the applications you are running can take advantage of them (or for multitasking, since you could run more programs at the same time).
I would investigate the applications that you use, and see if they are able to take advantage of multiple cores. If they are, then it is likely that you will see the speed boost, but if not, then there will be no increase in speed unless you are running multiple applications at the same time.
Are you rendering files and running a lot of compressions on your system? If so, then you will DEFINITELY see a fairly hefty improvement as long as those applications can take advantage of the extra cores. My guess is that your 3d application will be able to utilize the hardware, and an 8-core system would make your 3d renders much faster.
You might also want to look for comparison charts online. Several different blogs and websites compare the speed of different Apple computers using various software.