Could the high hand force needed at the lever be something unrelated to the STI lever itself, like sticky energy drink under the BB guide, or some other friction source?

In the past I've lowered the shifting force needed by changing a softer plastic BB guide for a genuine Shimano one that seems to be made of harder plastic.

Here's a pic:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=230380347490

From the description there:
"A worn cable guide can hamper shifting just as much as old cables."

I'm guessing your new LHT doesn't have a worn one but if it's a different brand or type of plastic maybe it's worth trying a Shimano one?