I installed the i7-3770T along with the Noctua NH-L9i cooler (with the low noise adapter). It works really well. I can't tell if it is any faster but it makes a lot less noise.
The only noise I hear is the cabin noise due to the negative pressure. There are a couple of intake inlets on the bottom but they aren't big enough to suck in enough fresh cold air therefore the negative pressure. Installing a couple of small 40mm Noctua fans before these bottom inlets can help reduce the negative pressure and reduce noise. The ASRock H61 itx motherboard has another case fan output which can be used for this. Sound dampening on the top cover is also a good idea.
I also replaced the stock SSD and HDD with a 128GB and a 1TB Samsung 950 Pro drives. I used a Corsair dual SSD bracket to install these (stacked) on the right front side where the original 3.5" HDD was. I replaced the stock SATA cables with Cablemod ModMesh cables. A red one for the 1TB media drive and a blue one (McIntosh blue) for the OS/Sw drive. These changes might help slightly with the air circulation however mostly for cosmetic reasons (better cable managed). Lastly I swapped the stock 2GB ram with 2x8GB Kingston RAM just because.
For the CPU/Cooler upgrade:
You need to remove the stock cooler and CPU. You need to apply thermal compound on the new CPU. You need to remove the motherboard to secure the new Noctua Cooler to the back of the board then install the board back to the case. All together, I got all this done well under half an hour but I have build many computers before. If you have never done this before, it is ok if it takes you a couple of hours. I would say the difficulty level is 6/10.