I drive a Lancia Delta Integrale 16v 1990 so of course I had to give the Lancia Delta Evo in GT5P a good listen with the volume turned up high.
Some things to note though: My car is a Delta 16v with 3" exhaust system and an open air filter, and not a stock Delta Evoluzione like the car in GT5P.
In real life, the Evo has a bit different exhaust note than the 16v. Especially on idle it's a bit more 'gurgling' than the 16v.
Anyway, this is how similar I found the Lancia's sound in GT5P to be compared to the real thing:
The interior view - Not very similar I would say. In GT5P you hear the idle exhaust note, in real life you really don't, at least not in the way you hear it in GT5P. The engine sounds isn't that similar either on idle. It gets less similar when you start revving. Sounds way too generic, and when you quickly depress the throttle all the way, in real life you get a bit of growly intake roar. In GT5P, none of that.
Exterior view, looking at the rear - Pretty similar on idle. Has that 'gurgling' kind of noise that the Evoluzione also has in real life. Start revving and it once again starts sounding a bit too generic in GT5P. In real life, the Evoluzione has a kind of boring, but sort of distinctive exhaust note when you rev the engine. At least a sound that is easy to remember. Actually, forza 2 nailed the exhaust note on the delta evo pretty well. GT5P, no.
Now I held the triangle-button to view the car from infront and listened to the idle, and yes! That's more or less what a delta sounds at idle. You clearly hear the injectors working, just like in real-life

Start revving, and it's a bit generic again.
Overall, in GT5P the delta is kind of growly, especially compared to the lan-evo IX. In real life, the delta is pretty growly too, for being a 4 cylinder turbo. But the growl in GT5P isn't that similar to the growl in real life. It's too hard in GT5P, in real life it's a bit more metallic-like, or how to describe it.