I have the 2007 4s Yamaha 150 on my 2007 2120 and it runs fine with a full load of fuel and passengers. Now turning 38 MPH and topping out at 5,300 RPM and getting close to the 100 hour mark with a ss Yamaha prop 14 1/4 X 16 that I had cupped.
Yamaha has a Reliance prop that I want (about $500) but am waiting for Parker to test it to confirm my buy. It should be a ss 14 1/2 X 15. I believe this prop on the 150 4s on the 2120 would be a real champ and should turn up over 5,500RPM (I'd like between 5500 and 6000 RPM WOT). Yamaha designed the Reliance series primarily for the 150.
Before I bought my boat, I looked at the bigger HP motors which were heavier in weight. The extra cost for the bigger engine and to hang maybe 50 pounds or more on the stern of a stern heavy boat made no sense. And it only yielded a few more MPH on Yamaha and Parker tests. Both recommended the 150 and I am glad they did. Yamaha also "suggested" that the 150 had their best and latest technology.
And all the buyers I am seeing today want a 4s engine. It us super quiet and has no smoke. .,,,,,,,,,,,,,,,,,,,,,,,,Pete