Greetings,
NVMe is not natively supported on Z97 (Gigabyte) boards.  This support has to be in the BIOS.
Been a while this this was discusses or asked. (Bear with me)  
 If I recall, another forum member here added support for it in a custom BIOS.  
Ya...  
regit…  
 https://forum.giga-byte.co.uk/index.php?topic=16870.75Maybe reach out to him?
I personally do not use modded BIOS, but as long as you know the risks, that's up to you.  
Using Add-In cards....  Can be hit or miss.  Some might work depending on chipset, BIOS rev, and others might not.  As you said however, a boot delay is likely during the detection phase.   
I don't personally recommend personally don't use modded BIOS, but at this juncture, the Z97 is old enough now that