I tried Tamiya yellows because they were suggested by someone that knows a heck of a lot about TC setup and is probably the fastest driver at the local track. They worked pretty well, provided excellent grip, but during tech inspection after the first qualifier I was told my chassis was touching the carpet too much on the right side (layout had a couple high speed left hand sweepers), so I switched back to TOP 306 rear and 367 front. Lost maybe a little bit of grip, but my chassis stopped hitting the rug at least and any loss of grip wasn't really that noticeable.
The guy who suggested the softer springs has a narrower chassis that is balanced much lighter than mine. Until my new kit arrives, I just have the original chassis and mine weighs 1427g balanced.