The Bosch Super's should help with the rough idle.
As to the sluggishness, I have three easy recommendations.
1. Be sure to use premium grade fuel
2. Check your fuel filter and replace (easy DIY) if in doubt on age.
3. Run a bottle of Techron Fuel system cleaner with a full tank of gas on your next long (hwy) trip.
That is if your 420SEL drives like it is thirsty, and not getting enough fuel delivered.
Good luck, Michael