Shilo, just look in a Lyman manual and compare the 255gn loads for .375 and 38-55 using RX7 it is an eye opener. My load for my CB is 29.5gns of reloader 7 with 250gn jacketed from Stone Fence.
Comparing the 38-55 and the .375W is easy both are very similar in size and case capacity.
The 38-55 case is larger not by much but still larger in length and dia, so it will hold more with equal wgt bullets as the 38-55 will be slightly shorter and larger dia. The .375 case is thicker reducing the capacity more but offering more case strength, kind of moot when you are putting them in leverguns. Back to the 38-55 attributes, the 38-55 has a slower twist, there fore has less resistance when pushed by the same pressures exceeds the .375 in fps easily still not by much. But when you buy this brass click link, and get the original length case which is .050 longer than the W stuff put out today and at $82 per hundred not that bad.
http://www.buffaloarms.com/browse.cfm/4,209.htm Couple that with a modern firearm such as the Marlin CB you can load it up pretty stout and at the same levels and beyond the stats for the .375 because of the slow twist and larger bore.
Ed Swanson