The big difference between KJF Pac first and KGF Atl first is that Japan can and usually will activate war early, whereas Germany will almost never activate early (assuming that London cannot be lost so quickly when played by any competent player).
That means that if you build Pacific first, those units can deploy and be useful as a deterrent and threat-projection whereas the units build in the East have to sit there and wait until US4 to actually move out, and then only to take Gibraltar.
One of the more interesting suggestions I have heard is to feint your intended direction during US 1 and US 2 (building the intended pacific fleet off the east coast) and then on US 3, move the entire thing to SZ 11 or SZ 28 and add it your US3 buy. The fact that America appears to be KGF may cause Japan to over extend (at sea, past SZ37), and then have to scramble backwards to meet a big new threat. However, this whole movement is way too slow to surprise much of anybody.
The inherent problem with KGF in my opinion is that Germany is too well defended, with too many critical objective territories that you need to take (Norway, Gibraltar, Rome, Normandy, Denmark, etc), and you only get 1 wave of troops to accomplish your initial goals in Europe. If you spend money on warships and planes with the US, they take forever to get into position, and your ground forces are too weak. If you don’t, Germany has enough planes to threaten to wipe out the entire stack.
You can dance around the periphery, wasting time (with the US in Africa or Malta etc.) but all the while, Italy and Germany can produce 20 infantry for less than a single turns income and use already existing planes to threaten your beachhead. Italy only has to protect one square, a square that is well within striking distance of Germany (as are all the other potential targets). Its very easy to “slam the door shut” by blocking SZ 112 or SZ 92.
If you have a truly lordly stack of land units by US 5, then you could potentially take Denmark with the UK; based on the rules this is one of the most important territories to protect because the US can threaten Germany itself. But when this goes down you had better have a truly lordly stack, since at that point Germany vitally only needs to protect one of two territories in depth (either Denmark or Germany) and then, against a power that has to use stratty bombers for its offense (never fighters; they cant reach the key territories without other footholds).
Defending the Atlantic Wall is one of the few things that Germany or the Axis can do while simultaneously sending virtually everything against Moscow.