Are you sure it's going to be named DB5? This I haven't heard of.
I heard of the AMV8 Vantage name for the smaller car (although I know this is to be changed). The DB9 will be a sub-5 seconds to 60 M.P.H. car, which is enough to keep up with the 911. When the figures are that close, it's the ability of the driver that makes one car faster than the other, not the engine. Perhaps on a straight road the 911 might nudge ahead, but I don't know that many straight roads. It depends on what you want to draw parallels with.