First off, I'm guessing you're referring to ported top-end, including the Injector Housings (IH) and port-matching the heads to the IHs - not simply porting the plenum alone, yes? I've not seen dyno results for porting ONLY the plenum, and am guessing that is why this thread has gone begging.
Second, the evidence clearly demonstrates that top end porting of the LT5 does not fit the 'mold' when it comes to low end torque, post porting performance. Apples n oranges = 2-valve vs. 4-valve dynamics (or so it would seem in the SBC vs. LT5 anyway!), especially when comparing HP results!
The power made by the DOHC, 32 valve LT5 is such that low speed drivability and performance is not sacrificed in spite of making just over 500 chp.
Stock bottom 350 LT5, stock cams, stock Throttle Body, secondary throttle plates deleted, fully ported primary and secondary runners "somewhere
above 36mm", headers, X-pipe, 3" exhaust...all with a stock idle!
I do so wish I had dyno'ed the car, prior to porting the heads; just for the sake of having recorded it for reference. But, I can tell you it added significant
seat of the pants grunt in the mid range too - just the opposite of all the
one reads in car rags - (I'm guessing wheel HP around 400ish, which compares with similar known examples.)
Here in FBI land you've several sources for top end porting, e.g., Marc, Pete, Lee Gaffgahn. OR, you can do it yourself (I and others that have done their own can put you onto the source for the tooling you'll need: just say the word!)
IMO, the ported version of the LT5 is how they should have come from the factory!! It completely transforms the demeanor of the car...for the better. And, the best part of all...
without sacrificing reliability, 
driveability!