I doubt it.
Although it's possible the name could have been shortened on paper to 'Sylla' by some US immigration officer or other, it seems unlikely as it is so far from the full name 'Syllwasschy', as written or pronounced.
The original Hungarian spelling is Szilvassy. This was corrupted in the German spelling to Syllwasschy, but it is still pronounced the same way ('Silvashy'). If the name were to be mis-pronounced and then mis-written in English, the tendancy would be to say 'Silvoski' or 'Silvaski' and write it as such (As some of my famliy used to do).
I think it is more likely to be a German abbreviation/corruption of another European name (the Germans are just as fond of re-spelling foreign names they found hard to pronounce as the English/Americans).
I hope this helps, but you really need to talk to an expert on German-American genealogy.