Apologies - just had another look
GA do manage to get all 4tph Stansted Express via Seven Sisters, albeit with extended journey times, in addition to 2tph Cambridge (so 6tph GA in total). Repeating half hourly Down pattern is:
Enfield
Stansted
Cambridge
Cheshunt
Stansted
Enfield etc...
(i.e 2 trains run behind the Enfield service that gets out of the way at Bury St Jn, with 1 train following the Cheshunt which is in the way for longer)
To get it to work, Hertford East only runs to/from Broxbourne, and Bishop's Stortford-Stratford doesn't run at all.