With a cam like that (and I'm not suggesting it's 'the wrong cam spec' , but rather it's just a matter of getting the rest of the combo right to get it to work harmoniously) you're going to want to run enough compression to make it 'work' You'd certainly want to be over 10.5:1 (based on the assumption you'll run pump hi octane unleaded, not e85, or else you'd be able to go much high) probably as much as 11:1 given that it's the smaller bore a12 (which also helps avoid detonation vs a larger bore). Given that that is the case, I'd second the suggestion of going for a closed chamber oval port head. They'll outflow the round port head in std form, so if you didn't want to pay for porting at this time, it won't cost any more to reco an oval port head than an a12 round port head).

With such a rise in compression ratio you'll tend to 'get back' a lot of the part throttle crispness, throttle response and even fuel efficiency that is often seen to go hand in hand with modified engines. As long as you can source the various jets and chokes for the dellortos, and enough time is spent optimising the chokes, emulsions and jets/air correctors, you can actually get pretty decent economy out of the italian side draught carbs in general (don't forget they were fitted in various forms to a bunch of smaller Italian cars in the 60s and 70s, although outside of Italy most people only saw them used on race cars and factory high performance cars. A lot of that was also to do with price. But there's no reason you can't get good economy out of that style carb and an engine combo like you outlined. Obviously the other factor is a little harder to control (and I'm as guilty as the next person!) and that is the fact that this engine will outperform a factory a12 quite well, there's going to be a constant temptation to drive it to access that performance. So the fuel efficiency will also be strongly tied to whether or not it can be driven in a manner that will result in fuel economy in the first place :)

You'll be fine running a 2 inch exhaust system. they always tended to work on 998cc-1275cc minis even though on paper teh smaller 1 litre variants ought not to have needed quite that diameter exhaust, they just never lost any power, and for some reason the larger engines also similarly just 'clicked' with a 2" exhaust. The a12 isn't identical by any means, but in practice it also tends to show the same result. It's a handy size as most off the shelf extractors/headers come with a 2 inch main outlet past the final collector anyway.