No the £350 million is wrong. Firstly it is a figure that the UK has *NEVER* sent rebate or no rebate. It is currently way less than £350 million without the rebate. Secondly there is no dictation on what we send the rebate on. So aside the £350 million figure being wrong even without the rebate, with the rebate it is even more wrong and misleading. Sure we would have control over the majority of the money, but actually a significant chunk is going to have to be spent on replicating things that are taken care of by the EU, and putting in new customs arrangements (think a lot more like 5000 extra staff).
Also while we theoretically gain control, in reality at least for some considerable period of time we will have to keep the same levels of spending in the same places as the economic dislocation of not doing so would be ruinous.
Of course BoJo does not care, because he is sufficiently wealthy that it matters not to him.