Until we're in a situation where the majority of teams are comfortably spending the SC every year a simple increase is unlikely, therefore we need to make the most out of SC exemptions to keep top players in SL.
The marquee allowance is likely to be to attract a star overseas player rather than preventing a Josh Charnley or Kallum Watkins moving to the NRL or RU. I think there needs to be more incentives for producing homegrown talent, perhaps a certain % of a club-trained player's salary is exempt based on number of appearances (e.g. 5% exempt after 50 appearances, 10% after 100 etc.), as well as similar exemptions based on England caps. This would at least give the club that has invested in a player since they were 15/16 year old some ammunition to make them a better offer to retain them should they start to receive bigger and better offers from other clubs/codes.