this will probably seem like a real newb question but. ..

lets say i cast an acid stream drain{ +1(dam +1)} at deadly.

Now in the main book it does not specify that deadly +1 is anything other than deadly, but in MitS pg 54 it says: "If the modifiers increase the drain level to deadly, any additional drain level modifiers increase the drain power by +2."
In this context it is talking about spell creation drain codes, so would this also apply to acid stream cast at deadly. Or is acid stream cast at deadly the same as acid stream cast at serious.

I think the +2 power goes both ways. If you cast the spell at Deady damage, you take Deadly drain with a +2 power increase (in addition to any power increases that the spell normally has).

Kind of like how firearm damage works, I believe.

It adds on. With that drain code, a spell cast at 4D would have 5D drain. at 6D, the drain would be 6D, and at 2L, the drain would be 2M.

On p.191 of the main book it covers drain modifiers. If a drain modifier raises the damage level above deadly, it adds two to the power of the drain. So a force 6 acid stream cast at deadly would have a drain code of 6D.