Got mine done at plus plastics as well. Before that i filled in the gaps on the old ones with sikaflex and they survived 3 years of drag racing like that.
you can use the 2 pak windscreen stuff on BRAND NEW mounts to re-inforce them to help them last longer. this also stiffens them up.
you will not be able to repair or improve worn/rooted ones though.
they will not be as stiff as a urathane one though.