I just have to state my long-time amateur view, with which many pros agree - and now I can give a first-hand summary which I experienced.
My son-in law, has been concentrating on putting for the past 6 months or so.. and completed the first 9 holes at 5 under par - one of his best 9's to date.
Disappointingly, the weather didn't join in the fun, and they were sirened off course due to lightning threat.

We have a Putt-Kew green in our yard, and play the pool-like game regularly. I seldom, yet sometimes win a game lately.

Sure, the synthetic green is not 100% the same as a bent grass green, but certainly stands in as an affordable almost maintenance-free alternative.

Here is a little challenge - and it will cost you nothing more than a little effort and honesty:

Place 7 balls 1 meter apart, and 10 meters from the hole on a practise green. easy enough.
Now putt the balls - and count your strokes.

Get them all in within 18 putting strokes, and you qualify for the game.
Get them all in within 14 putting strokes, and you're on putt-kew par.
Get them all in under 14 strokes, and you're doing good - really good!

Try it - and let me know the results here. It's simple, realistic and serious practice.

Get 32 of your fellow golfers to join your effort, and I'll send you details to play a competitive game and get one player to achieve a putt-kew bronze medal for one of the participants. OK, for the REAL bronze medal, there is a charge of ZAR150-00 per participant (About $20-00 at this time) for 32 players, but you can play the game 'mahala' (that's isiZulu for free) in your local practise green (depending on their policy)

Let me know please!