Yeh and with PDs history, They need a Game that would crush F5 and compete with F6, due to labour time! If F5 is Nxtbox then Pd need a Ps4 game, when ever that is....
(PD must have that Dev Kit 2.0)
If you actually look at the history (which you suggest), it tells a different story:
The average development time for the Gran Turismo main titles (1, 2, 3, 4, 5) is 3,4 years.
The average development time for the Forza main titles (1, 2, 3, 4) is 3 years.
Including secondary releases, for the Gran Turismo series (1, 2, 3, 4, 5 + Concept, HD, Prologues, PSP) it's 1,7 years between releases.
For the Forza series, including Horizon, it's 2,6 years between releases.
Conclusion: PD being slow developers is a myth. The main titles has been produced at pretty much the same rate as the Forza series. Including secondary releases, PD's production rate is actually 50% faster than Turn 10/Playground Games. Of course it can be argued that the secondary titles are much smaller games, but to PD's defense it must also be mentioned that GT5, which has the longest development time, was started from scratch, while other games (both for Turn 10 and PD) were based on the earlier releases.
If GT6 is based on GT5, there's no reason to expect the development time to be much longer than for the Forza series. If Turn 10 decides to start over for next generation of Xbox, then the result may well be the opposite, with a GT6 and GT7 before Forza 5 i released.