I personally think that browser mining could be a great replacement for advertising if done well. I read not too long ago that The Pirate Bay was running a miner too. The reaction here is not a million miles from that.
The real question is not whether it's permissible, it's really cool, and there's no legal requirement to publicize usage (yet, see below), but whether or not it's done ethically. I have to side with most people and say that they way you did it was not ethical.
The one thing you said above but did not address is that it's for your profit. It certainly is. I see the real issue in people's minds here is that it's a reversal of miner beneficiary, and it's follows the pattern of malware. It will have a negative impact on computer performance for the user, though they may not notice. You are requiring them to pay for access to your website in a way that is not totally up front.
The notice needs to be loud and clear. For example, like the European cookies notices, it needs to be always visible until intentionally dismissed. This insures to a basic level that it is read and noticed.
The best way to run an experiment like this is to allow user to opt in to the service in someway. It could be a "mine wall", similar to a pay wall, that has a full screen notice blocking entry unless you accept the terms, which allow mining. Or it could be a premium version of the site which uses the miner. Either way if I were you I'd be less defensive about it and admit you dropped the ball slightly, especially since it's now a business for you.
Incidentally I would expect a ruling from state, international and federal bodies on this kind of thing too if it becomes popular, perhaps a ban. So it might be wise to stay 100% above board.
RE: A Few Clarifications About Steem.Supply Browser Mining