I've been thinking about this quite a bit, and actually, merging the workers makes them weaker/inferior at building things...
It's due to the way workrate overflow works- when an improvement is 99.9% done, and only needs 10 more "units" of workrate to finish, a worker split down to 1/9th size and one merged up 9 times can both complete it in 1 turn. The unit finishing the improvement will then have used its "move"- meaning that the 1/9th scale unit only spent 1/81st the effort of the 9x merged unit to complete the same improvement.
So, if you're actually looking to get the most work out of your workers, you should always split them up.
It also raises the question of maintenance costs... I forget, do workers follow the normal Size Matters rules for maintenance, where a 3x merged unit costs less maintenance than 3 normal-sized units? If so, then it appears splitting workers up gets you less wasted effort (overflow on improvements), but at the expense of higher unit maintenance costs...
I'm not sure I like any of this, of course, because often in real life the effect of many workers cooperating on manual labor projects is multiplicative rather than additive- for instance because 6 workers can carry heavier loads (such as timber or big stone blocks) working together than 2- who would have to waste time/effort splitting up things to be hauled into smaller pieces/chunks...
Regards,
Northstar