Well, I am not expecting much from speed buff but Niji with suit did call Sanji's speed slow. Could be that with suit he is faster, we will see. Yes, Invisibility and Durability are the strong points of the RS as those two matter the most. Acceleration device supposedly increasing kicking power has yet to make a difference, maybe there is more than we saw, maybe not. I think the suit gives him no stamina buff at all, dunno how that would even work. If you meant Endurance, I guess so...
But Zoro doesnt want to use all his Haki at once, that is bad and it is not like he couldnt put all into one attack before. You can see that those mountain slashes have exhausted him noticeably. Besides, where would he need bigger slashes than against a mountain?
Yes, a sword gives defense but Shusui was a better defense, Enma only replaces Shusui. Yes, it gives him attack power but Shusui already did that too.
Clima tact doesnt train your haki.
Which part of this is confusing you?