Thanks for the info Nick. You have somewhat confirmed what I had calculated and I am thinking 3 HS4s will be adequate for a 2.6. For the 2.6 I had calculated ~255CFM required at 6000RPM (not that I would ever run at 6000, but I used it for the upper limit), and had an HS4 flowing 145CFM. This is slightly different than your 133CFM, but either way with 3 of the HS4s I get 399cfm or 435CFM which should be adequate for a naturally aspirated 2.6. In your case where you are only using 2, the swap to the HS6s makes sense, but I think I would be OK unless I am missing something.
The one thing I am still not sure of is throttle response for SUs. For down draft carbs, (which is where my experience is) if you run lower CFM carbs that still meet the upper RPM range, you typically get better throttle response than running an oversized carb. However, I am not sure if this holds true for SUs.
Jay