Not quite as simple as that. You can't simply compare an average fly be pilot on 400 ish hpa with an average fr/esy pilot doing 800 ish hpa and say the problem is productivity. They are not really doing the same job. Most fly be sectors probably average out at 0.55 to 1.30 ish. Ok I know they do some longer stuff but a lot of it is domestic around an hour stuff. Where as easy / Ryan will average 1.30 to 2.30 per sector with some canaries flights thrown in as well. Ok I know there is the odd dub floating around but looking at averages.
As most airlines do 4 sector days you can see even working flat out fly be crews will be building up less hours simply due to the type of flights they operate. You need to look at the number of duties they do than hours they fly to really judge productivity. So saying just get the crews working harder is easier said than done. Even if you moved to say six sector days I very much doubt there is enough extra business in the fly be sector to make it cost effective.