I too miss the dirty big laggy stage 4 turbos, with nothing but lag and then just an explosion of torque.
Reason that i imagine these werent included, is turbocharger technology has changed alot in the last 6 years. Back then ALL turbochargers were 360 degree thrust bearing at best, The bigger the turbo, the more weight was in the shaft/wheel assembly and the more weight was resting on the bearing creating more friction. basically meaning to get big power you needed big lag. As the maximum a turbo could flow increased, the lag exponentially increased. Hence the big laggy t88's ect Today ball bearing turbochargers have completely changed the game.The ball bearing cores make them alot more responsive, but also allows them to spin at a MUCH higher speed, being able to offer a significantly higher flow from a turbo with a much smaller frame, You no longer need a stupid big laggy turbo to flow alot of air. Take for example the GT40 series turbochargers from garrett, on a 2j they are capable of flowing up to and even over 900hp on the right housing, But they are just as responsive as the standard twins.
Or even a mildly sized gt3082 on my 1j powered soarer. The turbo is only just touching on its efficiency range at 22psi but im laying down nearly 450rwhp(with its efficiency range reaching from 20psi to about 40psi there is alot more power it could deliver), and on full boost by 2800 rpm. about 1000rpm earlier than the small twins would make a maximum of 9psi.
Im guessing PD have sort of looked at this when designing the turbo upgrades for the game. All said though, there is alot of room for more power than the current options are offering. It seems many aspects of this game were dumbed down. Infact it was quite a facepalm moment for me when I saw the options for turbochargers had been dumbed down to "low rpm", "mid rpm" and "high rpm". It feels alot like they have shifted the target audience from car nuts to kids/casual gamers to try and get an extra slice of the pie.