20 minutes of test are really not enough. I didn't find a significant increase in spam: if I see correctly, some more inactive accounts were created and 2 userpages, which had not been previously created, were added by two IPs. Let's collect information on https://www.mediawiki.org/wiki/Extension:ConfirmEdit/FancyCaptcha_experiment... We can tweak the abusefilters and repeat the experiment another time.
Nemo