Here's a little video for you
It was pre-beta, so behaviour might have changed since. It gives it another 20-25 mph or so on top of the pedestrian 250 mph the Ia can do normally. It's pretty good, but not too overbearing. One of the main problems though is that it won't overheat as long as the rad is fully open. The rad should work a bit more efficiently at speed if partially closed whereas the opposite seems true, so I'm not sure if they're modelling airflow and overheating correctly.
Basically, you can fly around forever on max boost as long as the rad is open. Pilots were under strict instructions not to overboost for more than 5 mins IRL IIRC due to wear and tear. At any rate, the fully open rad is too efficient IMO.
The boost is engaged at 50 secs in the vid.
SLIGHTLY OFFTOPIC: There is an entry for reliability in the engine setup for all of the engines:
Reliability Lifetime 100 h FailureProbability 0.0
Be nice if they could do something with that in the future to simulate old/badly maintained engines.