My two cents is that you're not going to find an "expert" anywhere that can give you a conclusive yes or no answer for the Hemi 5.7L. A lot of folks "feel" like 5W-30 may protect better in the heat but FCA apparently didn't come to that conclusion during their testing of 5W-20. Apparently GM came to the same conclusion during their testing of 0W-20 for the 5.3L EcoTec engines, because that's the only grade they recommend for all conditions. The usual response is that the only reason FCA and GM and all of the other manufacturers of trucks sold in the US have been recommending XW-20 oils for many years is CAFE, and that is part of the reason, but I believe their testing has shown that it provides more than adequate protection under all operating conditions.
The manufacturers have forced the oil companies to develop oils that meet their requirements, which includes wear protection while still helping provide good gas mileage. GM has led the way when they began requiring oils meet their Dexos standards before they can be used in their vehicles while under warranty. Some of the GM Dexos standards are tougher than what the API has. The API volatility test that's known as the Noack test requires oil not lose more than 15% during the test, but the Dexos generation 2 limit is 13.5% and will drop to 12.5% when generation 3 goes into effect. This test is a good indication of how well an oil holds up under high heat, so any oil that displays the Dexos symbol on the front of the container has met that standard. Testing and certifications are all that really matters.
People can, and do, argue all day about whether or not 5W-30 protects "better" in the heat than the recommended XW-20 but really can't provide any hard data to prove it. I firmly believe that if FCA had found that 5W-20 didn't provide adequate protection for the 5.7L they would've chosen a different grade. After all they recommend 0W-40 in the SRT, which apparently is what their tests found to be the best grade for that engine. If they were strictly interested in CAFE they would've went with XW-20 or 30 and hoped for the best. FCA recommends synthetic 5W-30 be used in the 2.0L turbo in my Jeep, again because their tests found it to be the proper grade. It all boils down to whether you believe that 5W-20 was chosen because of FCA's testing or you can spend a lot of time trying to find a genuine expert, and not a hack like Project Farm, that can show conclusive evidence that a higher grade works better.
Some manufacturers do recommend moving up a grade under certain high stress conditions, like your Nissan. Here's what I found to be what I believe is the recommendation for your car:
"Mobil 1 (0W-40) (100% synthetic) is the factory fill oil. The VR38 engine with its plasma-sprayed bores was developed
using this oil. NISSAN cannot ensure proper engine operation and durability if other synthetic oil is used. If Mobil 1 (0W-40) is not available, Mobil 1 (10W-40) (100% synthetic) may be used; however, some performance loss may be noticed.
Furthermore, replacement of the engine oil with MOTUL NISMO COMPETITION OIL type 2193E (5W-40) is recommended for the frequent high-performance driving opportunities. NISSAN cannot ensure proper engine operation and durability if another synthetic oil is used."
10W-40 is an odd weight, I'm surprised it's still even made because I don't know of any auto manufacturer that recommends it as the first choice oil. It does have some use in older vehicles but 0W-40 is so much better it really serves no purpose anymore and 0W-40 has become the factory fill on some of the highest horsepower engines out there, like the SRT and the Corvette among others for a reason. It's probably the best oil being made right now. If I ever felt the need to move away from using 5W-20 in my Hemi, which I won't, I'd skip right over the XW-30 oils and go straight to a 0W-40.