Apple goes to great lengths to ensure their portables run quietly, albeit at the cost of increased operating temperatures, however what you can be assured of is that Apple equally puts a great deal of focus on the efficiency of the cooling system as after all they want the operation of the machine to be as imperceptible as possible.
My take is had the OP posted before and after numbers this post would be of far more value, not to denigrate his efforts. You can achieve the same if not better with a software solution by increasing fan RPM`s, at the cost of some noise intrusion, to the point of reducing temp by as much as 20C. Punching holes in the bottom case is something i need to think about, first thing would be a second base plate as the original carries the machines serial number, and would effect resale.
As for the fans they are basically pulling air, the exhaust is sealed so in theory a ventilated base place should make a difference as we see in the Retina, albeit a more engineered solution. A fair start would be calculating the area of the Retina`s air intakes, and as for the vent holes any local machine shop will be able to "mill" out any shape you want at a moderate cost, taking a hand drill to one of my Mac`s is just not my style
I just purchased a 2011 mbp 15 that I am using with ext. display.
With just surfing safari I get 53 celsius at 2500 rpm. Just add Skype video chat I am at 78 celsius.
I would like to know what temps. did you have with ext. display and did you experience any problems in the long run by drilling holes?