5N LA-EA2 or E zoom - help!

I bought the 5N at the beginning of the year with the kit 16 and 1855 lenses and think it is the most amazing camera - for fixed focus shots. I previously used - and still have - an A700 with a Tamron 18-250 and the standard Sony 18-55 and 55-300 lenses. It is a terrific camera for action shots - lots of horse jumping and running dogs - but compared to the 5N, picture quality and low light performance are in the dark ages.

So, if I want to use the 5N for the action shots, what is best - the 55210 (I can't justify the £650 for the new 18200) - or the LA-EA2 with my A mount lenses. Will the quality using the A mount lenses be better than using them on the A700?

Don't get me wrong, I love the A700, but I am now spoilt by the 5N performance. Or maybe I'm just spoilt....

Any advice from someone who has been through a similar dilemma would be greatly appreciated.