It IS possible to SLIGHTLY improve performance on a stock 235 by going to 2 carburetors IF ONE DOES IT PERFECT!!!!!
It is also quite possible to significantly LOSE PERFORMANCE if one does NOT do it perfectly!
On a stock 235 (no other modifications) the change could be from a minus 25 percent to a plus 10 percent, depending on ones homework and skill. Is it worth it? Each will have their own answer.
The following link will explain further:
http://www.thecarburetorshop.com/Dual1barrelcarbs.htmAnd just because some company offers something for sale is absolutely NO guarantee that the product is beneficial to anyone other than the investors in the selling company!
And just to expand slightly on a comment by Jerry (HRL): we find that we virtually ALWAYS have to RICHEN the jets when going to multiple carbs. This is because of the reduced air velocity in the carbs. The novice will not account for the action of the power valve (which will need adjustment), and mistakenly thinks the carbs are too rich.
Jon.