I can personally attest to the problem of positive base pressure causing oil seeping at a seal, as the large engines at my job are designed with a vacuum system to avoid this very problem. I have no experience with the breather system upgrade you are asking about, but did you check to see if the stock breather valve was sticking due to oil gunk? A faulty breather valve will cause positive pressure in the base leading to leaks and also cause an oil sludge build up over time due to the exhaust gasses not venting properly...including moisture vapor...which will lead to a very thick gunky build up which can cause things to stick, suck as breather valves. Also as the engine has a few hours on it, it may be experiencing high blow by, which will also add to the contaminants getting past the crankcase breather. I would run some marvel mystery before an oil change and use carburetor cleaner to thoroughly clean the crankcase breather valve before I would go any further.