This is exactly what I have been doing, using FreeFly. I have not even done The Heist yet ( and I am not sure I want to) and you are entirely correct. I have found I can now drive anywhere, all round the map system and engage enemies (but many drop items will be unusable due to your char level and enemies are much tougher) . I have found you cannot drive back into Watson as the lockdown is on both sides of the bridge with road barriers and cops, but you can still Fast Travel to anywhere in and out of Watson from anywhere else outside, just not get back in or out by road. I imagine you could just drive to the bridge then fly over the blockade. I think you can probably even fly your car or bike over but I've not tried that at this location (it works everywhere else) I even brought a motorcyle up to put into V's Mansion lobby.
I am ignoring the main quest line as much as possible and i'm concentrating on doing any and all side missions. I figure that if the game ends after you finish the main story I may as well get as much fun out of it as possible
I did't know about this bug but I heard that (obviously) the time that it take to complete the main questlines is shorter than the side quests, so personally I'm at 200 hours played, been doing a lot of side missions and fought crimes here and there and still have almost half map of those to do. Naturally I don't do just quests but I also spend time to look at stuffs, places or details here and there. As for the main missions I've only done those of the initial part of the map because you are forced to do them in order to unlock the rest of the map and I cleared all the other missions in that area.