Another "raised from the dead" thread bump, but I did want to throw my 2 cents into this ring as I just went through a similar dilemma myself, and searching long and hard for answers to questions I had, it often lead me to these forums.
Answers here are great for the next guy who has similar questions.
So here goes; after all my digging, this is the conclusion I came too. There's an old "rule of thumb", not 100% gospel persay, but a pretty good and reasonable baseline to consider regarding exhaust setups, and it goes roughly 0.5" diameter of pipe for every 100hp you're making, give or take. Again, not 100% gospel, but not a bad place to start for most applications. Especially a truck, rather then a high performance sports car.
The 3.7L produces 210hp stock
The 4.7L produces 310hp stock
(Give or take), That being said, not getting into long details of airflow, combustion ratios, etc. In simple terms, the 4.7L can definitely benefit from an upgrade to a 3" pipe, especially if you are adding airflow & hp elsewhere, go for it!
However, if someone has the 3.7L like I do, and are considering exhaust options, your best bet is to stick with the 2.5" piping, because even if you're adding other mods like cold air, throttle body, tuner, etc, outside of something truly crazy and custom, you're only adding a few ponies here and there where you can, but you're not going to max out the capacity of the stock 2.5" piping w/ the v6 motor, especially since you are already replacing the stock exhaust with a free flowing aftermarket one, which will be less restrictive in and of itself. You actually look to risk losing performance by going bigger.
So take it with a grain of salt, FWIW, but if you have the 3.7L engine, stick with the 2.5" single piping, maybe split it into duel 2's via a Y-pipe to even the flow (2.25" max, but that might even be pushing it?) if you want something custom, because just a new less restrictive design in and of itself, it'll handle whatever you throw at it from this little v6, even you do manage to get it upwards toward 300 hp. Some folks even have 2.5 on 4.7's and it handles the 310 hp from the stock 4.7 just fine, even with little mods, and honestly the factory pipes is actually a decent set up all things considered.
If you have the 4.7L however, and start adding mods to increase flow and HP then you definitely could stand to benefit from going up to a 3", or duel 2.25" or even 2.5"s.
Shoot, even a 3.5" pipe might work on the 4.7, if your also doing all the other mods + a tuner + wrapped headers with a bigger Y to bypass the restrictive stock manifold to cat downpipes, but that's only if you're really modding out the v8 in it's entirety and start passing that 350hp mark by a good enough margin, and start getting up closer and closer to 400.
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