According to the above referenced article, yes it is normal. I would imagine it is for full throttle enrichment as you mentioned. If your control plunger was leaking, your fuel economy would definitely suffer ... noticeably. You could also encounter fuel dripping from tail pipe.
Replacing the plunger seal is a very DIY job... I do not know if your need to recalibrate after replacing though ... I guess that depends on whether it was leaking or not.
You may try to adjust the lambda slightly in either direction without a DMM to see if your throttle response and idle clears up. 3mm hex key turned clockwise will richen mixture, counterclockwise will lean it.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi