I have taken two pairs of gill magnum choice stainless riser pipes and extended to about the same length as bravo wet tail pipes,[a couple inches shorter].I needed to do this cause nothing would fit under gunnels except these risers. I will be running kamma silent select exhaust tips so I have to use 4" exhaust hose to join tail pipes and kamma tips. Now the problem. I was told by previous owner that these motors had loppy idle so want to make the pipes as dry as possible without burning the exhaust hose and mufflers I was thinking of drilling small holes at tip of inner pipes to let some water in and add dumps at end of outside pipes and out through transom fittings.Has anybody here done anything like this? any body know what size and how many holes to drill into inner pipe end. Thanks for any info

Are these currently dry or wet?

What you want is a common option on lots of performance exhausts (called "reduced water").

It's hard to know how many 1/8" diameter holes are required to keep the hose from cooking. Depends on your water flow volume, which is tied to other factors.