I think the only good reason to go up to a heavier oil in the modular 3V/4V engines would be operation in hot climates or a lot of hard work. Lots of tow/haul work, for example, where the oil temp goes high and stays for long periods under heavy loads. I can't speak to a 4.6L in a car, but I monitor engine oil temp on my '05 F150 5.4L 3V and it seldom reaches 210/212 (the temps for the hot viscosity rating in case you didn't know), even on a long freeway drive in hot (for here) weather. I haven't done any long tows, at least over about 20 miles, but have done some short, very heavy tows (loaded grain trailers) and oil temp does not rise much above 209 over about 15 miles on 80+ degree days. Now that cold temps are here it runs about 185 on a long haul. At around 90F, it runs 200-206F.
In the end, if you look at the good to high quality 5W20s, you will see they are between 8 and 9cSt at 100C (212F), which is at the high end of the grade. If you look at UOAs, you will also see that they are generally very shear stable, so they stay in grade. Many 5W30s are at the low end of the 30 grade to start and a lot tend to shear down a little (or a lot) at the end of an OCI, some into 20 grade or nearly so (of course SAE 30 is a pretty narrow band, isn't it?).
Bottom line, I don't see a lot of gain going to a 5W30 but not a lot to lose either. If you live in a hot climate, you'd have a little cushion to account for heat, especially if you used one of the better 5W30s that don't shear much.
You hear of a few folks running 10W40 or 15W40 without any disclosed problems but, personally, I don't see any need to go above the SAE 30 grade in a modular without some verified reason... like french fry tank oil temps. I haven't been able to learn what the viscosity threshold is, but the cam timing advance mechanisms and tensioners are reputed to be sensitive to oil viscosity. I don't know exactly what happens in that case. In a hot climate, probably very little, but imagine a 20F 15W40 cold start in an engine designed for 5W20!!!