The biggest downside to any exhaust system is that, all exhaust systems, even straight pipes with no muffles, will send a revers pulse back up the pipes to the engine. These reversion pulses are actually pressure waves (not actual gas) that can easily travel against the direction of the flow of the exhaust gas. The large majority of these pulses are created when the exhaust system changes size, up or down or direction. There is a pulse created where the header meets the pipe (Unless they are the same size and properly joined.), another anytime the pipe changes direction (Bends, elbows, etc.), another where the exhaust enters the muffler and again where the exhaust leaves the muffler and finally where the exhaust leaves the tailpipe. Resonators create them and even a long enough
straight run of pipe will make them.
All this being said, dumps can make a difference in power. However, the difference they make depends on several factors. The biggest difference is on engines with cam profiles that promote weak cylinder exhaust gas scavenging. The reverse pulses make the scavenging much less effective, causing a dirty intake charge, which in turn results in reduced power when the piston fires.
As a general statement, I would think that an engine making 1.28 hp/inch with only a 3.75 in stroke would benefit from dumps. They should however, come directly off the header collector in order to eliminate the restriction of the exhaust pipe. They should also be as close to straight off the collector as possible. They will be illegal loud when open.