I have seen it go both ways. On one hand, Germany does have to commit a lot of resources for Sealion which leaves them weak in other areas. On the other hand, a successful Sealion deprives USA of an airbase for attacking Germany. If Axis is able to protect Gibralter, USA has no place to base operations unless they attack neutral Spain, thus making all other neutrals pro-Axis.
It really depends on how players build in their respective countries and on naval victories. One key I have found for a successful Sealion is for Germany to build an aircraft carrier with 2 fighters to protect their transport fleet. If UK tries building expensive planes and warships to try and rebuild the RAF and Royal Navy, Sealion can be successful with somewhat limited units by Germany. However, if UK just plops a bunch of infantry there, Germany has to commit a lot more land units of it’s own which could keep dragging it out.
Another thing I have noticed is that Sealion really needs to happen G2 for Germany to be able to still build up a decent force for Barbarossa and protect against USA. In our first game of Global, Sealion wasn’t successful until G3 and so weakened Germany on other fronts that Barbarossa ended up being a failure. Germany kept having to pour so many units to keep at Russia that USA landed in Europe, liberated France and destroyed Italy. After that, it was just a matter of a couple of more rounds and Germany was finished.
In our second game, UK did a little poor planning and Germany pulled off Sealion in G2. As a result, they had plenty of force for Barbarossa and smashed the Soviets. Soviet counterattacks failed and the Germans pushed onward. By the time USA could get to Europe, Germany had too much defense and the US even had trouble fighting Italy.