As long as your 6/3 Romex is 6/3 + ground, then it looks fine. That said, I think you could save some money and have an easier time with it if you ran your 6/3+ ground directly to your 50a outlet, and had that as your only permanently installed circuit. The welder does not need the GFCI panel.
Then you can make the GFCI panel a "pluggable" device that can plug into the 50a outlet. If your control panel only needs 30a then you can put an additional 30a breaker in the spa panel (see http://www.homebrewtalk.com/f170/has...-panel-397746/), and run 10awg to the spa panel then onto the control panel. This is probably the most cost-effective solution.
Or, you could run 6awg to the spa panel and then to the control panel, and have breakers in your control panel like BadNewsBrewery did: "(6ga) from panel to spa panel to control panel. 6ga hits contactor, goes from there to 4 distribution blocks (Hot A, Hot B, Neutral, and Ground). 10ga from the Hot A / B a very short distance to 2 internally mounted DIN 25a breakers and 1 15a breaker. From those, appropriately sized wire to individual components."
P.S. This also offers the advantage that you can take your GFCI with you if you ever need to move.