Hi Alex,
I don't have / I don't know the 2014-15 RS6 but all 2014-15 Exocet RS are riding very (very) flat. New 2016 shapes are very different for that.
For my 2015 RS4 and RS2 I'm riding with :
- Footstraps to the rear position.
- Mastfoot at the rear position (129 cm).
- A fin with a high lifting capacity is better, but for that part you don't have a bad fin (Z-Fin)

But for that reason I'm not surprised that a longer S1 doesn't improve anything.
- You are probably riding with high pressure on mastfoot (like I do) and this doesn't help with such a board and with a big heavy sail.