A "code compliant second floor addition" requires one thing to start...an engineer certified plan as part of a complete building permit application. Without the building permit, you will not have a code compliant second floor and will have all kinds of problems at resale. As far as what size lumber is needed for second story joists, its fairly easy to look up span tables and determine the lumber species, grade, and size that will result in a known deflection for a specified use at a specifc span distance and joist spacing. http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

2x6 lumber (No 2 str) 16-in O.C. spaced is generally adequate for a living space span (deflection 360) of 9-feet 9-inches. The span table can help you, but you will likely need plans approved by an engineer to pull a permit.

Let me rephrase my questions:

The spans are greater than that allowed with 2x6 for floor loading. It was fine as an attic space, but will not be sufficient with the new live and dead loads of an occupied space. I'm looking to see what least invasive methods people have tried when upgrading joists to 2x8's or 2x10's on a second floor without damage to the 1st floor ceiling.

I'm fully aware of acceptable spans, engineering and jurisdictional requirements.

I am looking for creative construction techniques to a second floor addition without excessive demolition.

You mentioned sistering 2x10s onto the 2x6s. Seems like that would be pretty noninvasive to the ceiling. Does it need to be any more complicated?