The DOT has a design guide that you may use to calculate the intake capacity.

http://isddc.dot.gov/OLPFiles/FHWA/015808.pdf

RFreund
Speaking from my florida experience, I, as well as many other florida engineers rely on FDOT criteria and inlet capacity tables.  I believe these tables are based on the principle of flow through an orifice and use an equation similar to an orifice equation.  I beleive this is correct, but I'd have to look at the FDOT document to confirm for sure.  At any rate, I typically use the tables.  I don't have the tables open in front of me, but my intuition tells me that you will need more than one 2' diameter inlet for 8 cfs.   You can probaly search for the tables on the FDOT website and access them.  I would also imagine that other state DOTs would have similar tables.

Weir flow (instead of orifice) is typically assumed unless the ponding depth is deep. Depth of ponding is the key criteria here and is usually limited to 6 - 12 inches or so. Also, you should consider that flat grates commonly get blocked with debris such as plastic bags, leaves etc and theis can reduce the capacity by 50% or more. Two foot round, flat grate is probably not sufficient.

The inlet design is discussed in this Urban Drainage Manual Page 21. You must also be aware of the regulations of the local agency that will be approving the project.

http://www.udfcd.org/downloads/pdf/critmanual/Volume%202%20PDFs/090%20Chapter%2009%20Culverts%202001-07%20rev.pdf

Neenah Foundry Co. has done actual tests on their grates, both in curb flow and ponding.  You should be able to access this data from their web site and select a grate that is similar to the one your ae evaluating.

bimr links the document I'd use.  It's long, but worth the read.

The answer to question 2 is easy - a two foot diameter grate is usually not enough for an acre and a half of parking lot drainage.  I've occasionally seen inlets designed that sparsely when the client is large, private, and cheap, such as an owner of an industrial warehouse facility who doesn't really care if their parking lot floods out routinely, but even for those guys you need to show clearly on your construction documents the limits of ponding for design storms to cover your own butt.


 

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

The Denver flood control manual (UDFCD) is fine but for more general information, go to the FHWA website for the original source material that is referenced by the UDFCD.  Such as: HEC 12 - Drainage of Highway Pavements or HEC 22 - URBAN DRAINAGE DESIGN MANUAL.