I Feel Goku and Vegeta would've definitely benefitted from still having their Tails.

Allow me to expand on this.

Early in DBZ Raditz and Vegeta tend to emphasize on the Sayians' Tails. Insinuating that their true potential is fully optimized given that a Sayian keeps said tail. Then they go on to talk about transforming into an Oozaru. Granted at this time they were all oblivious to The Super Saiyan Transformation and believed the Oozaru to be the peak to a Sayian's potential. After that, the rest is history.

In the end though they were still right. I personally feel like it's not talked about as much, but with that much said I feel that the Tails were vital in providing full mastery/control to the SSJ Forms, by Extension the Godly Forms.

The biggest example that stuck out to me was Super Saiyan 3.

We all know that as it currently stands SSJ3 is Goku's most Underutilized form because it's the most Inefficient to use. It offers EXTREME Strength, Speed, And Power Output, but it burns through his energy reserves & stamina VERY quickly. Goku even states that after a while in the Form while he powers up he's LOSING more power rather than gaining it. As it is now it's not meant for a drawn-out fight, but only for ending fights as quickly as possible given the right scenario.

I believe that if Goku had kept his tail, and spend a good deal of time training in this form appropriately, he would have lasted a GREAT deal longer against Kid Buu. Idk about killing Kid Buu, but the fight would've lasted significantly longer had he kept his tail.

Lemme provide an analogy to further explain. (And try not to take it too literal).

Imagine filling up a balloon with water to a great extent, but you notice that it as a pretty small hole leaking not that much water out to make a difference at first.

Now Imagine The Balloon being Goku's Body, the water being his Ki, and the hole being the scar where Goku's tail would be had he kept it.

Filling Balloon with more Water= Goku Charging in SSJ3.

Hole in Balloon gets Bigger and Leaks more Water than before= Goku loses more Ki while powering up in SSJ3.

Balloon Ultimately Pops= Goku powers down to Base Form.

Now Imagine placing a Piece of Flex Tape on the hole to keep the water from leaking from the Balloon and keep thr hole from getting bigger.

Flex Tape= Goku's Tail.

See where I'm going with this?

Granted the Balloon would still pop anyway after filling it with enough water, but now it'll at least be a good deal more durable now that the water is no longer escaping and the hole is patched up.

This is what having the tail would mean in terms of a benefit for a Sayian as strong as Goku or Vegeta. It may not exactly offer any sort of a power advantage necessarily, but more stability and optimization of certain transformations as well as aiding in more Ki control, and Endurance specifically as well.

Now I know in the Cell Saga Goku catches on to this and trains to minimize the stamina drain as much as possible and thus, you have Mastered SSJ. Just imagine him doing that, but NOW with a tail.

Hell, it could even be argued that Goku could've stood to Master SSJ FASTER after getting out of the Hyperbolic Time Chamber in the Cell Saga, and POSSIBLY gotten stronger than in the original story.

And the best part is that the benefits stand to only get more and more better with the more Forms/Techniques they get throughout the series i.e Kaioken, SSG, SSB, Blue Evolution, MUI & U.E. and so on.

But Yeah. With all this said, I think the Tails should've stayed.