IMHO, the answer depends entirely on the use of the car. Assuming all other supporting mods are in place:
(1) If its going to be a circuit car then greater displacement is the way to go as you get all important torque. Hence the OS 3.0ltr kit would be ideal. Obviously, you'll lose the ability to rev high with this approach but that will be less important given the size of the turbos you would match to a circuit car.
(2) If its going to be a drag car then just stick with an N1 block and standard displacement but uprate all the internals to billet/forged items. You can then rev high and make use of the air flow afforded by huge turbos. An analysis of the specs on the RH9 car engines should support this as many of them displace less than 2.7ltrs.
In an article from one of the issues of GT-R Magazine the OS Giken kit has been tested to sustain 9,400rpm. However, Saurus (a tuning house that promotes the OS kit heavily and has worked with OS in testing turbo kits on it) recommends a maximum operating rpm of 8,500 rpm.
As Guy said, 10,000+ is possible on the 2.6 ltr dimensions. Veilside is a good example where they spin their rb26 in the R34 street drag car to 11,000+ rpms regularly. After all the 3240s like that sort of flow.
Sorry, I don't have any authoritative numbers on the max revs the 2.8 and 2.7 kits can sustain. However, what I do know is that HKS shift their drag cars (R33 and 180SX) around 9,500 rpm (using their 2.8 kit), and make peak power just over 9,000rpm with the huge turbos they run. This shift point also tends to apply with other tuners that use the 2.8 kit in their drag cars.
With 2.7 kits (eg. Trust, JUN, etc.) there is evidence of RH9 cars taking the engine to 10,000rpm. However, this appears to be the upper end. Tuners with shift points at 10,000rpm or later tend to keep their displacement below 2.7 litres (eg. stock stroke with a 1mm overbore piston).
Apologies if I'm not exact with the numbers as I'm going from memory based on articles in the Japanese mags.
Not sure. The only thing I know about the factory Nissan 2.8 is that it's used in the turbo-diesal Patrol 4x4. Not sure if that's built for high power. Sorry can't be of more help.
If you want to increase displacement with factory items, it's far more common to to use the RB30 bottom end from a Holden VL Commodore (sold in Australia) and mate it to an RB26 head. I understand this is a proven combination and the fitment is not too complicated. Finding uprated internal parts are also easier.
V1H I would have thought that the N1 blocks still the stronger of the two (I haven't heard of an off the shelf Nissan 2.8 with full tuning potenial) The N1 block is still used for the enduro series in japan. I am certainly happy with my choice since my stock engine was discovered to be porous after the previous owner had fried it!
I am still only at the early stages of tuning it but the results I am getting with the Reimax turbos at just 1 bar look promising for the future mapping and boost increase.