Like I said in your other thread I had the same rib with a yamaha 25hp outboard and it was a great match! However, with just me in the rib WOT was very dodgy unless the water was very very calm! You could easily flip it! I would go as far as to say that WOT with 30hp would simply be too dangerous with just one person in the boat! Stick with a 25hp over a 30hp outboard and your be fine!
That said I am sure a 15hp will "do the job" IF its just 2 people in the boat. Horses for courses!
If not mistaken the Merc 18 HP is called the Super Merc Pro or something like that, check Mer web page. If you have the option go for it, has 50 cc more to the younger 15 HP brother and with same exact size and weight.
We use 310-340 ribs with Tohatsu 18 HP engines, at wot with just driver very scarry when going from a dead stop to plane momentum.
A 25/30 HP is too heavy and too much HP for a 340 rib. Bear in mind that max stated engine on transom's plate indicate max passengers allowed too. If boating with just 2-3 a 18 will perform quite well.
BTW, New Sea Pros are same HP rebadged Tohatsu versions, being Tohatsu bit cheaper.
Northwind Challenger
However, with just me in the rib WOT was very dodgy unless the water was very very calm! You could easily flip it! 
I would go as far as to say that WOT with 30hp would simply be too dangerous with just one person in the boat! Stick with a 25hp over a 30hp outboard and your be fine! 
