I don't think it is asking too much but we must consider that XML is freeform and not static like a function call. Plus once the XML is parsed and validated this must be turned into a function call at some point. Either by an XML aware driver or some wrapper application.
On the plus side it would give the app developer the ability to batch up device related commands and send them over all at once. This may have the knock-on effect of better device utilisation.
If batching were allowed then the "XML preprocessor" will have to be that bit more complex to to decode and order the requests.
Its an old but true argument that as we up the level of abstraction the level of performance will decrease.
So depending on the device you are trying to interface with you may not see any perfomance degredation from the users perspective. Of course from a technical aspect. An instruction from an APP to write/read some data now has to go through another processing layer before reachingthe device; this will make a performance hit no matter how light-weight it is. It is up to you to define if this is acceptable.
I hope this helps, an interesting idea that I would be interested in talking more about.
