I know that as a German player in MP that I would be pushing my belligerance to the floor with puppet releases to avoid the USA getting into the war at all. Barring the planning of a first strike with Japan, the "strict neutrality" option would be my most likely choice.
Stand by for tangent;
As a USA player, I'd probably accept a 50% dissent hit just to get into the action in the early war, unless I was restricting myself to balance the game. If the USA intervenes in 1939 or early 1940, the game is over. The Axis can forget about advancing farther than France. The European Axis can give themselves some faint hope by going after Russia, but it'll be getting free supplies from the USA, massively increasing their production capability.
In the case of a Sealion getting pulled off, there may or may not be a free dow opportunity for the USA, depending upon the mod the players are using. If not, the Irish series you're working on sounds interesting.
Rather than simply avoiding invading the UK early (eg early 1940) because it heralds rapid USA intervention, the Axis could make a call on how to prepare for the inevitable American atlantic island-jumping campaign. If they wanted to push for a stronger, farther-reaching airbase, there's Ireland. They can even produce a decent handful of troops with the manpower boost of Ulster. And (unlike vanilla) its possible to just puppet/ally with them rather than annex-release. Of course, there's the possibility that the US will get to intervene (much?) earlier. This fits with how Axis plans in games that I play often fall apart in MP. You up your belligerance too much, you up American intervention too much or forget about an event, and boom. American intervention. (There are reasons you don't DoW Spain and Sweden!)
American intervention almost guarantees the end of the Axis advance anywhere but Russia (where it is slowed). This is probably something an Axis that has defeated the Home Isles, has Gibraltar, and at least threatens Suez can afford to risk. Lets make the game a bit more interesting, right? (Sidenote: I'd recommend that Greenland and Iceland be transferred to USA in the first round of the Harsh Terms UK peace) But the war's still going to be balanced against the Axis. The RN is still around, a good amount of manpower is tied up guarding the western islands and the IC spent on the planes and ships for Sealion isn't really transferrable to the eastern front. Its still going to be tied up trying to guard against an invasion from Iceland.
Were I the US player in this scenario, I'd say my best bet would be to send an unholy amount of supplies to Russia, letting their full available production go to unit builds, even letting their lines stay on permanent offensive supply.
I'd then make my way up the east and west african coasts, taking on Spain, Egypt-Sudan, and the middle east. Once bases were available on the spanish coasts or inside the med, any garrison savings made by Sealion would be lost, and Germany would be in even worse shape than before it took the British isles.
End tangent.
Through my own experiences, a hefty dissent penalty is appropriate. If the German player screws up and can not pull off an invasion quickly enough to destroy Ireland's resistance, the penalty could certainly be less. Against the AI, the game doesn't need much balancing.