While I'm sure I could find this somewhere if I looked myself (*cough*)
are there numbers to hand which would give the capacity of the current 7
car trains (standing and seated) against 7 cars of the new Crossrail
stock, and then the 9 cars of the Crossrail stock?

The existing trains are two 4 car class 315 units running in mulitple.
Therefore 8 cars. The loadings are so bad people can't board trains and
TfL Rail has recently starting banning entry to llford station with
people forced to queue outside due to platform overcrowding.

I'm keen to know how much extra capacity will be unlocked by the move.

(it did pain me a little when I found out the actual number of trains
along the Shenfield line won't significantly increase post Crossrail,
though the lengthening and improving of the rolling stock will make a
decent difference, even if it's only short term before it fills up)

Does Wikipedia not give some capacity info for 315s and 345s (CR
trains)?

Wikipedia had some of the info, but I did find this elsewhe

https://www.gov.uk/government/upload...tachment_data/
file/4153/tpecapacities.pdf

Class 315 (data is for 4 cars, so doubling up)

636 seats
226 standing
862 total

Class 345 (data is for 9 cars, rather than 10 - is the 10 car set up for
later?)

450 seats
1150 standing
1500 total

(which is a pretty significant total increase....)