There is a strong argument that the £250M should have been used.The reason IMO while this case will fail, is that the 350M claim, is pretty much the amount that the UK is due to pay under EU rules. (VAT, GNI, Customs contributions etc): £357M a week. The figure after the rebate is 250M, and after all other grants and funds is 170M. There is a strong argument that the 250M figure should have been used. All other grants and funds are tagged for specific projects, so they could not be spent on the NHS for example.
All of the above was widely reported and challenged during the campaign. The 350M was a "sexing up" of the facts and we have seen plenty of MPs and PMs sex up facts to support their case. e.g. WMD in Iraq. We then have high test of misconduct : https://www.cps.gov.uk/legal-guidance/misconduct-public-office I don't think a campaign full of sexed up facts amounts to misconduct.
To use a higher amount was not 'sexing up'. It was then and now a proven inaccurate figure. That's not 'sexing up', that's deliberately misleading.
To use the WND/Iraq argument as an example of sexing up and comparing it to the £350M claim is not relevant.
The WMD/Iraq argument was an argument based on probability, intelligence, etc, not on proven facts.
The £350M lie was a distortion of proven facts. It was not an accidental exaggeration, it was not 'sexed up', it was not based on probability or suspect intelligence.
As you say, using exaggerations and 'sexed up' information may not amount to misconduct, but to knowingly use the wrong information could amount to misconduct, if the person was in public office, and if his opinion could have an influence on the outcome. But that is for the court to decide.