The AT300 design is supposed to be flexible, but there will be constraints just as there were for 390 extensions (8, 9 and 11 car were possible but a 10-car wasn't).
390 9 cars are DM+M+PT(&Transformer)+M+T(removed-on-8-car)+M+PT(&Transformer)+M+DM. Each transformer vehicle can only feed 3 Motor vehicles. 11 cars require an additional Trailer/Transformer (without the pantograph) and a motor vehicle. A 10-car unit should be theoretically possible but it would only be a trailer so would be lacking power (60% motored vs 63% on an 11-car and 66% on a 9-car).
80Xs supposedly can only come in an odd number of vehicles (5,7,9,11??), I'm not sure why.
5 cars is good for off-peak, and 10 cars is loads for peak services. EMR can't justify 9/10 cars all day, and a mixed fleet would be more difficult operationally and would require a much larger depot to deal with longer units.
In addition I watched a Hull Trains 802 (303, according to RTT) pass through Luton Airport Parkway yesterday afternoon and it was definitely in diesel mode too.
Anyhow,
EMR issued a presser on the 810s today. There's also a YouTube video below.
Looks good, vast improvement over the IET interior.