Its 8tph from the Thameslink core and up to 6tph from Moorgate.
14tph peak sounds easily sufficient to me. (This is excluding the Cambridge Superfasts)
Stevenage would get both the Cambridge and Peterborough trains, giving it 4tph and Peterborough has access to a large number of ICEC trains making two trains per hour probably sufficient.
4tph on the Inner suburban trains is almost turn-up-and-ride frequency, and this excludes anything from Moorgate which could feed another three trains into the mix.
You could easily reduce it to 2tph Inner Suburban through Thameslink and make up with the Moorgate services, leaving those extra two to bring Peterborough up to 4tph and 6tph to Stevenage.
Is 8tph peak or off-peak frequency? I reckon that off-peak is probably going to be the same as now, giving a bit of flexibility in the timetable to sort out delays. During the peak, it's simply impossible to ram anything more through Welwyn, but the xx:14 and xx:44 peak extras would most likely go to Thameslink, which pushes it up to 6tph. Additional peak extras might push it up to 8tph, but that would depend on capacity between Wood Green and Welwyn. Can WGC handle 4tph terminating? Otherwise, it's a choice between absorbing the Beer Train or transferring it to Liverpool Street to free up paths.
Of course, there's also the question of where the trains go south of Blackfriars. I'd say one each to Brighton from Cambridge and Peterborough, the rest going wherever convenient. Still, we'll know in six years, I suppose.
As for using IEP on this, I know that Baker wants to do so, since he assumes that there will be sufficient acceleration to cope with the stops on the Fen Line. The thing is supposed to be able to out-drag a Pendy, 0-100 in less than 100 seconds.
Yes, I forgot that. Maybe we'll have to go with The Ham's idea.