Once HS2 phase 1 is completed, journeys from NW England and Western Scotland to places between (and including) Reading and Taunton, and Devon and Cornwall, will generally be quicker via Old Oak Common (and even quicker if HS2 extensions north of Handsacre are ever built).
Bristol to Paddington currently takes 1hr 35m. Euston to Curzon Street will take 45m (we'll count the OOC-Euston timings as our interchange penalty). So Bristol to Birmingham via OOC will take around 2hr 20m. If anyone can recall the Bristol Superfast timings I would appreciate it but I doubt that it will close the gap on this.
Bristol to Birmingham currently takes 1hr 20m. So you could use XC from Bristol to New Street, transfer to Curzon Street, catch a HS2 Phase 2 train to Manchester and still be past Manchester Airport at the time that you'd be passing the Delta junction if you'd gone via OOC.
Obviously the transfer penalty between New Street and Curzon Street is quite steep which is why I suggest a chord from Curzon Street onto the Camp Hill Lines so that the HS2 Phase 2 services from Manchester/Leeds/Scotland can continue onto classic lines towards the South West as a significant upgrade to the existing XC network.
So Bristol - Curzon Street would take 1h20m as now, 10 minutes for reversal, 40m Curzon Street - Manchester. Total 2h10m. Current route for comparison is 3h. Via OOC would be 2h42m.
Similar for Leeds if the Eastern leg ever happens. For Taunton add 42m to the XC times, 10m to the OOC times (not the Bristol Superfast ones, the 1h35m to PAD ones). That makes them equal on the Manchester route. That's all - OOC would be equal to XC, with the upheaval and risk that changing trains brings (compared to a one-seat ride on XC). I have also not accounted for any gains made on XC's classic network by electrification.
Do you really want to be sticking even more people onto GWR's busy services - where there is limited scope to improve capacity - rather than XC where capacity can be sorted as fast as a proper rolling stock order can be fulfilled?