He's maybe feeling a bit insecure after having you all to himself for most of his life. Perhaps he noticed that the rough children got lots of attention - little children sometimes do things that get them attention even when the attention is a telling-off.
The nursery workers will have seen behaviour like this from lots of other children - they won't be judging you. I wouldn't have thought his superb cognitive skills are causing him to be bored - nursery is about so much more than numbers, shapes etc.
The arrival of a sibling often causes the older children to regress for a while. I'd be firm but undramatic with your response to his rough behaviour. Don't spend long talking to him about it - do the naughty step or whatever as matter-of-factly as you can and try to give him as much positive attention as you can the rest of the time.
I'm not an expert, but I'm sure this is very common. Having a toddler and a baby is sooo hard in my experience. This phase will pass
I have a 2 year old son, turning 3 in march who's behaviour has changed dramatically. Since the middle of November he seems to be rather naughty when socialising with other children, pushing children, snatching not wanting to share. Nursery has picked up on the sudden change of behaviour and that week when he started I was with my friends children who were playing rough with my son, kicking, pushing, biting and even standing on him! And now I think he has decided this must be how we socialise. He hasn't been mixing with these children since as I have gradually been trying to calm him when interacting.
I have noticed in the nursery room it is very basic for him, even though it is for 2 year olds. They have simple puzzles yet my son knows all his colours, recognises letters and numbers and can manage to do 45 piece jigsaws without looking at the picture. Im starting to get the impression he has a photographic memory not just because of the jigsaws but because of other things he is doing. He can also play a memory card game on the iPad with up to 40 cards very easily. I'm starting to think maybe his behaviour is due to frustration on being bored in nursery room?
I've tried the naughty step but it doesn't work, he gets a row and cries but within a few minutes he will do it again. He is so head strong and no is not in his vocabulary! I've noticed when he is playing sometimes he is actually just hugging very rough and I don't know how to get him to be gentle. I repeat it constantly to be gentle but it doesn't work.
I don't know if this is typical 2 year old boy behaviour or if he is trying to tell me something!! But I'm starting to get scared to go out in public. I should also mention in September he became a big brother so there has been change!