It's hard to say whether Spieth would've enjoyed more success with Bones on his bag versus Greller. But two things are very much apparent: Bones has caddied for 25 years on tour and has much more knowledge regarding both the proper misses and the green-reading aspects than Greller.
Given Speith's putting prowess over the years, and given his need for both him and his caddie to be functioning at a very high level for him to win - Bones (I think) would be a difference-maker going forward.
And given Bones' admitted appreciation and respect for Jordan's talent over the years, it would just seem like a very good partnership all the way around.
Whether Spieth is the type of player who would dump a close friend for a more seasoned caddie? He doesn't strike me as the type who would do that. Greller is where he is primarily because of his friendship with Jordan, more so than his technical expertise as a caddie. Greller (imo) isn't that great of a caddie. He most always refers to Jordan regarding club selection and target selection... Jordan's talent (particularly back in the early part of his career) has made Greller look a lot more competent than he really is, imo.
But I would be shocked to see Spieth dump Greller nevertheless. Which, imo, speaks volumes as it relates to him appreciating those who've stood in his corner, despite his own level of play in major championships in recent years.
But do I think Bones would give him a huge boost of confidence? Absolutely.