I’d have to confirm with Mike, but I think that since fares from Slough are set by GWR, the fare to Paddington (their London terminal) is inevitably cheaper than the rest of Zone 1 stations which have to share the fare with TfL.
So in event of through operations of Elizabeth line beyond Paddington the fare arrangement should be the same, that means fare to Paddington would still be cheaper than the rest of Zone 1.