So I bought my Cobalt and came into this predicament... the previous owner put Monroe Quickstruts on my '09 SS! They're brand new, like he said, but I'm building this car for SCCA autocross and need something performance oriented. Already looking to trade my new struts for somebody's blow FE5 setup, but if no bites, I have a few questions.

Can I put Koni cartridges in Monroe's?
Do FE5 springs fit regular FE1 spring hats?

 

Buy new struts.

They'll do more for performance than the new springs.

 

Assuming the Monroe's were built to the same specs as the OEM then yes you can put the Koni insert in a Monroe housing.

Yes FE5 springs fit the same spring hats as FE1

 

Yup, going with Koni cartridges for sure. But I need the stock FE5 springs first!
Do you happen to know dimensions of a Koni insert?

 

Nope don't sell them, ask Koni they should have data either on their website or available if you inquire with their customer care/service department.