I'm not sure about his political and social achevements at the time of SGYY because it's all a bit shrouded in mystery, what with SGZ bio's being different depending on who you read about etc, but I do know what Liu Bei represents today..
Liu Bei- with his brothers- is a god, or at least as close to the idea of a god as we can think of in the west without saying that he's the one supreme.
What's interesting is that even though Liu Bei was the emperor, the way that LGZ wrote SGYY made Guan Yu into a hero who was an amazing general- that's why you can see Guan Gong temples with Zhou Cang and Guan Ping next to him, yet you will not find any Liu Bei temples because, even though he's the god of hero's, he's still not as important culturally for the Chinese.
That said, everyone knows who Liu Bei is but they base their experience on SGYY, not from historical sources, because SGYY is almost taken as truely historical.
I think the reason that a lot of people think of SGYY as more historically accurate in terms of personality is because it's read by younger people in China and Taiwan than I think it's read over here- the legends are told to kids in school who are around age 10, so they dont question the personality which is represented to them- when they read SGYY it becomes like a reality where Cao is evil and Liu is good dispite the historical souces being conflicting.
I hope this helps- if you want to ask anything more specific, post it here and I'll see if I can answer you!