More likely to move on to the fasts once electrification is complete and new stock is in use, they'll be a complete recast of the timetable then. IIRC there is a diveunder being built to allow XC services to access platform 7 at Reading from the relief lines without conflicting movements, platform 7 is the dedicated XC platform in the latest plans I've seen.
XC will not be having P7 dedicated to their services according to the drawings I have. It is described as the normal route from the down main for Westbury direction services.
At the display/discussion on the station concourse yesterday, they reckoned XC could continue to use a wide selection of different platforms, including P3 as now, and P7 and basically any of the four new relief side platforms, depending on whether they were terminating or reversing. Depiction of certain routes as 'XC' on track layout diagrams, such as the up relief to P7 diveunder you mention above, wasn't supposed to imply they were the only routes XC could use.
However the layout also allows XC to use the 'main side' through platforms as now, northbound XCs from the south could reverse in P8 and head straight along the down main after reversing, or reach the down relief via the 'XC' diveunder/underpass. Similarly southbound, a XC service off the up main can also reverse in P11 and use the nominally 'up Westbury' route in the down direction.
IYSWIM...