Hi Jon,
A 5-hp motor is sufficient to push the CD16; the speed difference between a 5hp engine and an 8hp engine will be very slight. What the 8hp engine would give you (since they're all 2-cylinder engines versus single cylinders on the current 4/5/6 offerings (the Yamaha 6 being an exception...it's a detuned 8 )) is a smoother and quieter engine and you'd have a bit of reserve horsepower. This won't help you 'fight' the current as you're limited by speed through the water (in other words, if 6 knots is your top speed with the kicker, it's going to be 6 knots through the water whether you're in still water or pushing against a 3 knot current; the ground speed will sure change though!); however, it will give you some extra power to push into the wind or waves.
If you're going to primarily use the engine on the Columbia the Honda BF5 is probably ok; I personally don't recommend it for saltwater use.
I think, on a small boat, that keeping 'extra' weight out of the stern is always better for a balanced ride. The more you shift the center of balance to the rear of a planing hull the more the ride changes from a teeter-totter motion to a fly-swatter motion (like too many bricks in the trunk of the car); the bow wants to bounce up and down with every little wave. However, the 20 pounds or so difference between a BF8 and a BF5 is not really a big deal on a CD16 unless there's aleady too much stuff in the stern. The BF8 will be able to run at lower rpm (quieter and easier on the engine), it will be smoother, it will keep a higher resale value (percentage-wise) and will fit on a larger boat if you ever decide that's the next step (it's typically the 'kicker of choice' on a CD22), and its got a 20+ year history. The price difference isn't huge either (here it's $200). So unless you're really concerned about the extra weight, I think the BF8 is a better choice than the BF5.
EDIT...I was referring primarily to the BF8A Classic in the above with regard to weight difference (which I corrected above) and money. The BF8A Classic is the 7.5/10/8 that Honda has been building for so many years. It's only available in a manual (rope) start version, weighs about 78 pounds (dry) and has a 6-amp alternator (the 12-amp is only on the new "D" model with electric start).