We could spend a few days on this subject, but I'll throw this out:
I rarely use a single pattern camshaft because all FE heads need at least a few degrees of split to help out a less than optimal exhaust side flow. Some cylinder heads are really good as far as exhaust flow goes, including your BBM heads, the Survival heads, CNC ported Edelbrocks, etc. These heads will usually be around a 70-75% ratio and I normally use a 2-6° split for most street engines. Some heads have very poor exhaust flow, like a factory Tunnel Port head for example. I have used 8-12° splits in the past for similar combinations.
A few side points:
1. Adding exhaust duration increases overlap. This can either help you or hurt you. If you have a large volume intake port (think 4V Cleveland or FE Tunnel Port), extra exhaust duration (overlap) will help scavenge the intake side and help intake port velocity. A lot of overlap, however, can prove to not be desirable for a street engine...low vacuum, rowdier idle, etc.
2. It all depends on the particular engine that you're dealing with at the time. There are tons of variables in an engine build and its application, which can affect the cam choice. It's really difficult to make ballpark statements.
3. A single pattern cam won't necessarily help with low-mid range hp or torque. Again, there's too many variables to make a broad statement like that, and it's entirely possible to grind a dual pattern cam that will increase hp/torque in the low-mid range, increase vacuum, increase street manners, etc. It just depends on the application and lobes that are chosen.
4. Most cam grinders don't know the engine they're grinding for on an intimate basis. Engine builders do. I cringe when someone tells me that they called up a cam tech line and ordered a cam for an FE, Cleveland, etc. Most guys that answer the phone have never seen an FE engine, much less built one, or built 100's of them. It's very easy for a cam grinder to grind a cam, send it out, and the end-user be happy with it, solely on the standpoint that they haven't tried anything else as a comparison. When you get to try 100 cams, you get to zero in on what works and what doesn't. The guy on the FE forum who's complaining about a noisy camshaft will probably learn the hard way that most of the Comp Cams out-of-the-catalog hydraulic roller cams are gonna have some noise to them.
I know that probably muddies the water, but this is a very difficult subject to throw out absolutes on.
