Hi Michael,
I have a Westerbeke 30B Three, 27 hp, so similar.
Originally 13 x 9 12 3-bladed prop.
I upgraded to a 15 x 9 feathering max-prop; a great improvement under power and especially under sail. On a calm day, flat water, clean bottom:
Old prop:
2400rpm 4.4kn
2600 4.8
2800 5.0
New Prop:
2400rpm 5.9kn
2600 6.4
2800 6.6
Wow. And the ol' Iron Genny has no problems spinning this larger prop.
As you know, an Ericson 38 has a lot more waterline than us, so I would expect them to steam ~20% faster. And a dirty bottom will slow you down. And as for the people on board...well, out boat weights 13,000 lbs dry weight, so maybe 14k lbs. Ericsons are proportionally lighter than the 323; a 38 weights around only 15,000lbs dry weight. So with either boat, the weight of average-sized people is a negligible weight increase, and isn't going to effect the speed under power that much (if at all). But that extra 6 feet in the waterline...for ~1000 lbs weight...that is another story.
I race against an Ericson 30 (amongst others)...can't get close, most of the time. Still win sometimes - give me a broad reach and a bit of chop :-)