Scala and ART, android runtime I understand that Dalvik has a just-in-time compiler.
The android team is making ART, android runtime, which has an ahead-of-time compiler.

Effectively using Scala can increase code execution time but I am under the assumption that reported performance is only regarding the Dalvik VM.

What would the implications of using Scala with ART be?
You shouldn't worry about ART or Dalvik since you have no control over which one they are using. Assume they are using the slower one and go from there.

ART is far off anyway and even after its out as the default for android it will still probably be years before its the biggest chunk of the market. And ART is still in beta so who knows how good or bad it will be after they optimize/fix all bugs, etc. Do NOT worry about ART  right now unless it causes your app to crash.

Scala can be slower but if it decreases your overall development time then just use it. Scala's compiler is still evolving and the possible gap in equivalent code is shrinking.