You won't notice the difference in CPU, but you will be spending $450-$600 on it!
You are looking at $500 to $600 to buy a known good board with that CPU. While you can go from a 2.5 i5 to a 2.9 i7, this will make no difference in day to day tasks - only long video encodes. Going from dual core to quad core is noticeable, going from fast dual core to slightly faster dual core in the same processor family will barely be noticeable.
For day to day tasks, you can buy two SSDs and put them in RAID 0. For less than half the money of a replacement logic board your laptop will scream as if it were a brand new retina with PCI express SSD. 250 GB SSDs are $100 now so two of the plus a caddy to put the 2nd SSD in the optical drive bay is $230 total. With two SSDs in RAID 0, you will get almost double the read/write speeds of a single SSD - you will have nearly 1000 megabytes per SECOND read and write speed on your laptop. This is in contrast to the junky 43 to 85 megabytes per second you get now, with all the latency from mechanical spinning hardware.
Weighing the options, you can get either a 1000% increase in storage bandwidth which you are using nonstop for $230, or a 20% improvement in processor speed which you rarely use to even half capacity for $600. I know which I would go with!!
Save the money and get yourself a much faster machine by dumping the mechanical storage. When 100+ track logic sessions, large video projects, and your operating system all load in eight seconds or less you will never go back to a spinning hard drive - and this is all possible with a 3 year old dual core processor when the storage is capable of keeping up with the processor!