What are the costs/benifits of one method over the other?
However, as the diff becomes larger, the tradeoff shifts. I think we passed that point a while back but, since we were going to switch models anyway, I took some time to clean up some bits of the code too.
With your own patches you know exactly the reason and intent for each one - you wrote it! So you can rapidly figure out what to change to make them fit with the new release.
But in reverse you are going to have to grok every new change sent your way, and that it going to be a massive amount of work.
Some of it will be easy stuff: It's clear what they are doing, or it patches cleanly and you don't worry about it.
But then you'll run into more confusing changes and you're going to have a harder time. There is a human tendency to ignore that which you can not understand, and you'll have some of those, then a patch on top of them, and the final result is more work than you started with.
Are N+1 heads better then one?
The group is better then the average head.
Heads are worse then the best head.
I do not know however if Google makes reviewing the patches more time intensive then it should be, it all depends on the rate of progress, isn't it?
The downside is that you may miss some upstream changes that you do care about.