My BZ gen4 now has recirc mod. Giving split recirc, between inside malt pipe & outside malt pipe (via one lifting hole). This allows max flow at all times, even during the initial 15min grain bed rest.
Original recirc pipe goes to outer recirc, with valve usually left fully open. 2nd recirculation flow (pipe from drain tap) is to grain bed, flow being adjusted with drain tap. After rest, drain tap is set for max grain bed flow, without wort level in pipe rising (more than 1cm).
Maybe close down outer recirc valve a little, if faster grain bed flow is possible.
The total flow is now always either equal to, or greater than, the original system max flow. Which seemed limited by the recirc pipe diameter (rather than the pump). I'd say total flow, is now100% to 140%.
The 100% outer recirc, means grain bed temp can be maintained, without base temperature overshoots. And it help keeps any loose flour recirculating (till filtered out later, in grain bed after rest).
I can't see lifting the malt pipe, being helpfull.
If pump flow is greater than flow through malt pipe / grain bed. Lifting pipe, might just delay the pump running dry. But rather than solving the problem, might make it worse.
Poor grain bed flow, is likely due to a compacted grain bed. The effect of this, becomes worse where there's more flour.
Compaction is always going to be more of a problem, in systems with a tall & narrow malt pipe (rather than a broad & shallow pipe).
If the surrounding wort level (outside malt pipe), ever drops below top of the grain bed (& when level inside the malt pipe will be seen to rise), the bed is no longer 'floating' and will compact under its own weight, due to differential pressure.
That is the reason why 'fly sparging' keeps a floating bed, for good sparge flow rates.
Also, Graham Wheeler advises: at dough in, stir minimally, to just break up lumps, without knocking air from grains. Attached air increases grain buoyancy, giving a more open (less compact) grain bed.
Since doing mod; switching to gradual grain additions - a 1L jug scoop, with minimal stirring between; leaving a 15 - 20min grain bed rest (outer recirc only, while grain & flour hydrate); and absolutely no stirring after dough in, I've since, never have a stuck mash.
Previously I was doing a vigorous stir in, using a spiral drill paddle. Then stirring more every time it stuck.
My Weizenbock recipe (56% malted wheat which is huskless, so high in flour, but with 6% oatmeal husk addition), was previously a completely stuck, burnt flour disaster. With same recipe, but using the amended method, circulation & sparge now worked fine (though at only a medium-slow rate, through malt).
