I just did mine but I didn't have to go under the hood. That seems way too much work. I just pried them up from the top, starting at the thin rim part (toward the front) and worked my way back (toward the windshield). There are tabs on windshield side so starting on that end would be too hard and could even break the OEM vent. My hood vents are from AutoTecknic and they don't fit 99%. I'd say 95% but still looks pretty good.
2011 M3 ZCT DCT MW Bamboo Mods: CF Rear Spoiler, CF Hood Vents, CF Reflectors, CF Mirrors, CF Front Lip Spoiler, CF Kidney Grilles, CF Steering Wheel Faceplate, CF eBrake Handle