I think the difference in reporting pension expense is the following:
IFRS - the vested part of past service costs is amortised
the unvested part: amortisation is spread over the average period until the benefits become vested
U.S.GAAP - past service cost is reported under comprehensive income and amortised over the average service life of the affected employees
Yes, the formula is the same except under U.S.GAAP the 'Amortisation of transition (asset)/liability is to be added in exceptional cases, however I would rather omit this. And of course the actuarial assumptions can cause differences in the DB pension expense.
On p.209 of the FRA curriculum there is a useful summary table on this topic.