There's also some "non cheaty" ways though.
Old news, sorry. I believe from what I've seen around here before is because the PS3 hardware can't handle 2-16 cars going their full speeds and rendering everything for everyone at the same time. Hence the 150mph cap, which even at that speed still can cause texture issues. There are ways to exceed that cap though, gate launch glitches etc, but generally no car will go faster than around 150mph. Maybe if/when GTA online comes to PS4/Xbox one we'll be able to have a higher cap, but until then thats as fast as we're gonna go legitimately.
I've 152.07 as my highest, it's not a lot more than the cap, but it proves it's possible without glitches also.
Yeah, try slipstream on, catchup off in a race(dipping in, downhill section for example)...
I'm not sure if it's still my fastest online speed, but i have a legit 174.9 MPH in a 9F using the above legal tactic.
Off-topic: Never hacked/cheated/glitched, although I'm sure i have been reported for driving too fast a few time, so many times I'm sure R* knows me by name, and is sick of checking my garage for NOS cars, to no avail...
Sorry so many slow little wanna-be squeakers join my race lobbies, and then get so butt-hurt after getting smoked by me.... Not my fault.