IDIBRONCO:
Your observation rings true with me too. All that electrical energy goes to heat. It wouldn't surprise me to see precup temps over 400 degrees F during engine run, then dumping 14 volts at 25 amps ( over 300 watts ) into the plugs... can't say that's doing anyone any favors.
On top of that, the mechanical "work hardening" from additional thermal cycling just makes things worse.
I get the strong idea an educated human brain, knowing the physics behind glow plug operation , will make far optimal usage of them.
Cubey:
I think the biggest benefit you got by taking manual control over your GP is that you get to put ALL of the current available from your batteries into your starter motor during crank time, as you take advantage of the thermal inertia of the GP.
I experienced that if I dedicated the battery to the GP first, powering them seemed to warm the battery up a bit, so it could crank the engine a bit faster, and the whole trick is to compress fast enough to get to flash point.
Once the engine catches, then it's back to glow plugs a bit if it's really cold and the cups are still cold. Almost like goosing a choke. But as IDIBRONCO stated, engaging GP on a hot engine is bad news...and you nipped that in the bud.
Eventually, I think, piece by piece, I am going to end up replacing every bit of "high tech" on my old 1994 E-350 IDI with toggle switches. Next on my list is the E4OD transmission controller, which I want to replace with four toggle switches ( SS1, SS2, Torque Converter Lock, and Coast Clutch ) and a rheostat ( Line Pressure ). I can wire the switches in such a manner that if both SS1 and SS2 are both OPEN ( Overdrive ), it won't be possible to engage the coast clutch. There are just too many things to fail for a bugout vehicle here, and besides, it deters others from asking to borrow this thing, as no one else knows what all the switches do.