I did the same thing to my down pipes. Realized those studs are welded to the down pipes and can't be removed. Landed up cutting them flush and drilling them out. Put in some nice ss bolts with anti seize.
Montego Blue E90 335i
7" VRSF IC | OFT Tune | DCI | RB PCV | Secondary CAT Delete & CYBA 3.75" Tips | M3 Front | CSL Trunk | 19" LC Dyna Wheels