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As a user, I don't feel it adds or takes away from the experience all that much, but I'm inclined to think that a filled-in (coloured) CTA button is more prominent and therefore likely to be clicked more.
Particularly interested in answers that point to any research/posts that look at this, I've had a look but haven't found anything useful.
Seems filled CTA is better. Outlined button is often reffered as Ghost button, so if you google with that term you will find results.
https://goodui.org/leaks/booking-a-b-tests-ghost-buttons-but-their-hopes-fade-away-as-predicted/ https://uxplanet.org/ghost-buttons-in-ux-design-4cf3717334f8
Research on filled in vs outlined icons seems to suggest that the difference is insignificant. It seems not far fetched to assume the same would therefore be true for buttons.
However, it may help to look at what other companies are doing. From observing a couple of performant sites, it seems that the standard seems to be to use filled CTAs for primary actions and outlined CTAs for secondary actions. This would mean that at least they believe that filled CTAs should be more performant.
I've given some visual examples with annotations here:
