Firstly, let's cut the debate about what in-game manufacturer resembles what real life manufacturer. There are cars of a certain manufacturer in the game that resembles two different manufacturers in real life. For example, it's possible that Benefactor is the GTA equivalent of Mercedes. However, the Benefactor Surano has a VERY HIGH likeness to the Jaguar F-Type. There are suggestions of it resembling an SLS, but i believe only in body-shape. The front and back of the car look almost identical to the F-Type. Then we have the Lampadati Felon that looks very much like a Jaguar XKs. You could argue that those look like Maserati's, and the likeness in the name would back that up, but Ocelot have the F620 and Jackal, both of which also resemble Maserati's.
Honestly, there probably is/are very solid arguments and reasons why each manufacturer is a GTA version of a certain real life manufacturer. But trying to convince each other that cars look like certain cars would leave us here forever as they don't specifically resemble anything, they just look a BIT like this and a BIT like that.
Secondly, the in-game stats are specific to that car. A full Acceleration bar on one car does not mean it has the same acceleration as another car with full acceleration. A car with half a Top Speed bar doesn't mean it is the same speed as another car with half a Top Speed bar. It doesn't really make much sense, and i think it's just to give you an idea about the cars performance. I wouldn't pay too much attention to it personally.