I asked this exact question of the EWS API tech support people a few weeks ago, and their answer was that there were no current plans to deprecate V6. Makes sense, since the whole API will be going away - why bother with any upgrades at this point!
re: EWS >> adCenter conversion document - it looks really good on the surface, and is probably the best single piece of adCenter API documentation that I've ever seen MSN come up with (ironic, isn't it), but if you look closely you'll see that it is written from the perspective of "adCenter API function xxx = EWS function yyy" when what would be really helpful is the reverse (ie, show the adCenter version of each of the EWS methods). You'll also notice that, while there places where the cell showing the EWS info on for a corresponding adCenter call indicates that there is no corresponding EWS functionality, there are no places where it shows that adCenter is missing something that EWS has. Pretty suspicious, eh? Nothing is missing in adCenter?!
I was this close (>| |<) to rewriting a ton of EWS code to take advantage of the support they added for bulk sheets, but never got around to it. What a relief. I've asked the adCenter API tech support more than once if they have any plans for similar functionality (ever since adWords API added structure reports, which they're now taking away), and the answer each time has been "No." If anyone has found any adCenter API tricks for efficiently loading keyword information (not performance data, but attributes, bids, etc., regardless of whether there has been any activity), I'd sure like to hear about it because the only way I've ever found is the say way that EWS worked before the added the bulk-sheet functionality: one adGroup at a time. That's fine if you only have a handful of adGroups, but I load data for a ton of clients and there are close to 2mil keywords across 50K+ adgroups, and it takes a lot of time to do that load.