Its a huge risk. Moving the G3 Sealion to SZ91 gives the US 52 IPC to spend in SZ101 which most likely will be 2 BB + 1 Fighter. It can probably move 2 Fighters to Eastern US from the Pacific SZ to Scramble. Assuming at least 1 AC was placed in SZ101 in the first 2 US turns, your looking at breaking through 1 AC + 2 Fighters, 2 BB, 1 Cruiser, 2 Fighters to Scramble.
The Italian Fleet in a best case scenario will be able to land everything but its starting Transport and Destroyer plus 1 Sub. The US will still be able to absorb 3 hits before losing any ships and its first round counter included 4d6 for Fighters against all 3 non-subs plus 2d6 for the BB. Italy is going to need a high luck roll to sink much on the first round of combat, although it may be able to sacrifice its subs to sink the Cruiser on a round 2 attack and if real lucky cripple one of the two BB or the AC.
Basically the Sealion is going to have to do the heavy lifting to get through all of that on a G4 attack at SZ101. Any later round attacks aren’t reasonable as the US’s income and unit placement changes drastically.
It is tempting, however, to see that the Eastern US likely will have 1 Inf, 3 Mech, 1 Art, 1 Tank as its only ground units in Eastern US. Against that, you really only need 5 Transports to be 95% certain of a victory (1 Inf, 1 Art in each). 4 Transports is 83% Certain and 3 transports = failed Amphib in my book at 37% certain. Assuming the US places no land units on US1 through 3, you can probably adjust your Sealion accordingly for more offensive sea units to break through SZ101 and land the Amphib.
The other significant advantage is that by posing this threat to Eastern US, is that the US won’t be moving a carrier and 2 fighters onto Hawaii for the J4 attack there. In fact, it may entirely withdraw to the Western US from Hawaii which is very advantageous to the Japanese fight in the Pacific due to the advantages of a defending fleet versus an attacking fleet at Hawaii while being able to project onto the Western US after taking Hawaii.