I don't see an issue with 2 tiered systems. I quite like the declaration of Rome from 25 March. They won't be able to act as they please; and they will be dragged to plenty of courts I suspect (as is the case atm; finally I would say). It's just a way to create more integration for countries that want this and are ready for this. Let's call it premium club membership. I see it as a way to correct the mistakes of the past; they were able to join too soon. A bit like the Greece joining the euro mistake; shouldn't have happened so soon.
Are you talking about the recent talk about the Tobintax? Because okay that might happen; if Belgium backs down (I don't think this will happen though- NVA will have problems in the next elections if they do). You also have the European wide corporate tax system that Moscovi wants to introduce. Again I don't think this will happen. I would be quite content if it happened but am highly sceptical since they've tried to do something similar in 2011 and some time before that. Somebody will get cold feet.
I was mainly talking about legal transaction costs (but you can find other examples), because that's what I know most about. Not only for the internal market but for simple things like when you've married two women in two different countries. An efficient way to find in how many states they can sue you; which law is applicable... Stuff like that. Supranational I mean for things like the climate and such; more efficient. There are things that are better dealt with on a higher level; the European courts have done some stellar work on environmental issues (pollution doesn't stop at borders). The costs of different legal systems in different countries across the EU is very high (lots of needless costs of involving extra lawyers and others); harmonization etc is the key. The EU is doing some stellar work here, if you're interested in contract law you should really read this
http://ec.europa.eu/justice/policies/civil/docs/dcfr_outline_edition_en.pdf (it's a beauty; it's supposed to be a preparation for a European contract law).
Yes I admit to being quite Pro-European. I can't be bothered with the founding tenets, the non-discrimination principle is one of the most important principles in EU law; some of them would do well to read up on it.