Hello Alex and thank you for contacting us. I'm replying to your questions, as your friend Neil Capstick asked me.
I hope to cover all your questions in this mail so you make a good choice for yourself...

First and foremost - esters are a good choice. However, one of your choices was Motul 300V - but this is not a true ester based oil. It has been formulated with 25-50% hydrocracked components (well refined mineral oil actually). See this link: https://docs.google.com/file/d/0BxnjLrq ... edit?pli=1, and it is more marketing than technical excellence. So I'll skip it and will focus more on the other, much better choices for you.

Now about your application, it is the industry consensus that gasoline turbocharged vehicles benefit from a more stable viscosity in relation to temperature. It is in their nature to fluctuate more in temperature than a normally aspirated engine. So having said that, a high viscosity index fluid is what you want. Off course the more of it comes from the base oil and not from polymers, the better for your engine. Very high HTHS may prove itself detrimental in turbo applications that run cool by reducing debit through the turbo oiling line and creating power robbing back-pressure for your pump (do you want that?).

So here is the data:
Your thermostat is fully open at 91C
Your fan comes on at 95C
Above are your operating temperature margins.
Your recommended (factory) fluid is synthetic 5W30.
Going one grade higher (40W) will give you the same factory viscosity even if you run 15C hotter.
Having said that, if you run hotter than that, then you should fix your problem and not compensate with the fluids. You can also get a water wetter bottle to run in your system and/or get a buddy club high pressure radiator cap to make your system work harder.

Your increase in hp would qualify for a slightly better HTHS. Porsche and Nissan GT-R tuners like Stillen rely on Red Line 0W40 where factory fluid is Mobil 0W40. They seem to do very well with a value of 4 cP that is very stable across the change interval. Factory Mobil 1 is at 3.8cP and drops fast in service.
We sell this fluid as well but recommend it for heavy 30W applications.

So to sum it up you need a high viscosity index fluid with a HTHS of around 4.
That is usually found in a 0W50 fluid.
The ones we sell are:
Red Line 0W40 (it scores as a 40 because it's a heavier fluid..density is high at 0.9 but otherwise this would score a 0W50)
Mobil 1 Racing 0W50 (HTHS 3.8, TBN10) (we sell it but not listed yet)
Eneos SM 0W50 (HTHS 4)
Other high quality options would be Fuchs Titan Race and Joe Gibbs 5W40 (virgin base oil viscosity index for PAO is 200 and contains lower viscosity esters and additives that bring that down a bit) This is specially developed for Porches 911 Turbos.

Bel Ray might also be very good for your Subaru. However going to a Polyol ester fluid will give you more advantages that anything. If price is not an issue then I would use Red Line products as technically they are playing at a different league than anything else on the market.
Many advantages like polarity (affinity to metal surface), oxidation rate at high temps, Viscosity / Pressure (‘α’) coefficient, natural high viscosity index are not reflected by the product data. Bear in mind I'm talking from only technical point of view and not marketing. Oil companies will often seriously overprice some of their products, as they put strong efforts and tons of money to promote some of their oil "models" and in almost most of the cases this is done via "creating a novel". In most cases this is being done via their marketing departments and has nothing to do with the chemists and other "technical" teams. This is why sometimes the "not-so-famous" brands will give times better performance from technical point of view (which you really like) compared to their marketing promises. And again - if you demand the best - stick to Red Line and you'd be happy (despite I'm working for other oil company, that's technically the best after the lab analysis).
Let me know if you need anything else.
Regards,
Tudor