I have a street-driven supercharged 400 Small Block, which is running standard heads with 1.94 inlet & 1.5 exhaust valves.
I have the opportunity to purchase some heads with 2.02 & 1.6 valves.

Could anyone tell me if there would be noticeable performance gains from the larger valves?

Cheers  

More than likely yes as with a 4.125" bore and more specifically 400 cubes the engine is more 'under-valved' than a typical 350 breathing through a 1.94" valve - the increase to a 2.02" valve is slightly more substantial. The real gains would be in the other areas of the port that will best utilize the larger valve. By other areas I mean most often the seat and valve throat...

Not until a 2" valve reaches approximately .400" lift will be the rest of the port really be "taxed" with airflow demand - this means that the valve seat is of utmost importance to a street engine and this is where the flow will 'come from'...

So, if these heads have a poor seat and valve throat compared to your old heads you can plan on losing or not seeing any gains. Of course there may be other considerations to be made like combustion chamber shape and volume, port shape and volume, and possibly the condition of the guides, seats, or studs... etcetera, etcetera.  

Thanks for the advice.
I've decided to wait and stick to my original plan of eventually buying some performance aftermarket heads, so as well as larger valves, I know I'll be getting good port design, valve seats, chamber shape, etc.