The R07 has noticeably more...
The R07 has noticeably more material on the top of the head. This adds approximately 5 pounds to each of the heads versus the SB2.2. Nobody likes extra weight, but the added material should provide a large amount of additional stiffness for the rockers, which can potentially make more power. The grooves aid oil drain, but they also direct oil to the intake valvesprings, which are higher in the head and will see less oil than the exhaust springs.
Anyone familiar with race engines will quickly notice that the head-stud holes on the new Chevy head have been moved away from the combustion chambers. Some might even call it a "Ford-style" stud arrangement. This will definitely be a benefit, as the old stud location was so close to the cylinder bore and combustion chamber that it could cause distortion. Although we haven't seen the block, it is a good guess that the area that the head studs or bolts thread into in the block will be completely separate from the cylinder bores. This will not only ease distortion-inducing stress on the bores, but also create more room for the water jackets to provide coolant across more of the bores' area.
Since the SB2.2 was introduced in the late '90s, many teams have begun flooding the valve covers with oil to help keep the valvesprings cooler. Some say keeping the springs surrounded by oil also helps dampen damaging harmonics. To aid this, the outer rail on the R07 head has been significantly raised. If you pull your valve covers a few minutes after a run, the raised rails will catch the remaining oil and you won't drench yourself. Additionally, the oil drain-back holes are easily accessible to change out the oil restrictors for different sizes, which will aid tuning.
It appears that Chevrolet's engineers have also made another change to help racers when it comes to oil control. At the lower corners of the heads are galleries that lead from the block to the top of the head at the inside edge of the flange for the valve cover. There is even a groove for an O-ring at the topside of this gallery. It appears this will feed pressurized oil to the valve cover, which will have integrated valvespring oilers. Integrated oilers won't necessarily improve performance over existing systems, but it will eliminate exterior plumbing and simplify the system.
This hole is an oil gallery...
This hole is an oil gallery that will feed oil from the block to integrated valvespring oilers in the valve covers. Notice that it is also grooved to accept an O-ring.
Finally, the R07 engine will use a valley cover that is separate from the manifold, like the SB2.2. Also, the valley cover will be wet and provide water to the cylinder heads. But unlike the SB2.2, the R07 includes separate water inlets into the head for each chamber. The SB2.2 only had inlets at the corner of the heads, which made it difficult for engine builders to get cool water to the interior exhaust valves. As a result, many were forced to plumb exterior coolant lines directly from the water pump to inject cool water into the water jacket between the two interior exhaust valves. Between the relocation of the valves and the four water passages into the cylinder head water jacket, cooling should be much improved and the external plumbing for the coolant should no longer be necessary.
Here's a trick the Cup teams have been using for a few years that MBE has begun providing to other teams. Below is an example of a cylinder head for a Sprint Car, but it will work in many different applications. Owner Matt Bieneman has gotten rid of the valve seal by using an O-ring inside the valveguide. The advantage of this is that it allows the guide to be taller (equal to the height of the top of the old valve seal) without limiting valve lift. In this application, it equals approximately 0.150 more guide height, which helps stabilize the valve. Now Bieneman can use smaller valve stems, which cuts valvetrain weight, without sacrificing stability.