I've found it extremely difficult to judge Marketo's ability (or lack of) in processing large numbers of records, whether it's processing data changes, adding to campaigns, sync-ing to SFDC, or running through triggered or waterfall campaigns. Along with a lack of campaign prioritization or controllable flows, the slow bulk processing capabilities of Marketo is one my least favorite constraints.
I know it's not the answer you want to hear, but it very much depends on your systems (Marketo and SFDC) as to the throughputs you can achieve. To give you an idea, though, I have done a lot of bulk processing because we have migrated from a non-Marketo system to Marketo then from our original Marketo (with a custom integration to a CRM) to a "classic" Marketo-SFDC set up - a lot of bulk processing! What I have found is the following:
Data importing (avoiding a system backlog): about 2,000 per hour
First time syncing of large numbers of leads to a SFDC campaign: we get about 500 per hour
Data change (i.e. changing the value in a single Marketo field): about 6,000-12,000 per hour
In your scenario (2 data values being changed, then sync'd), it would appear to me that making both changes first then syncing both updates at once would make sense, but it may be that someone with better knowledge of how/why the sync works may argue.
Hope this helps.