In terms of the regional population, Guangxi certainly stands out.
Population of regions, 2010 census residents of autonomous regions, provinces, municipalities and special administrative regions, millions:
Macau SAR 0,6
Tibet AR 3,0
Qinghai P 5,6
Ningxia AR 6,2
Hong Kong SAR 7,0
Hainan P 8,7
Tianjin M 12,9
Beijing M 19,2
Xinjiang AR 21,8
Shanghai M 23,0
Inner Mongolia AR 24,7
Gansu P 25,6
Jilin P 27,5
Chongqing M 28,8
Guizhou P 34,7
Shanxi P 35,7
Fujian P 36,9
Shaanxi P 37,3
Heilongjiang P 38,3
Liaoning P 43,7
Jiangxi P 44,6
Yunnan P 45,97
Guangxi AR 46,03
I suggest that Guangxi as a region is not less important because it is an autonomous region which Yunnan and Guizhou are not.
Nanjing, Qingdao, Dalian, Xiamen are all cities of rich coastal provinces. True, the problem with the classification "coastal" or "inland" is that Guangxi is "coastal" (Beihai, Fangchenggang...) but not particularly rich compared to inland provinces.
For context, the provincial GDP per capita, in 2013:
Jiangsu - 12 047
Liaoning - 9961
Fujian - 9342
Shandong - 9094
And now compare West:
Shaanxi - 6893
Sichuan - 5240
Guangxi - 4939
Yunnan - 4050
Guizhou - 3701
So... in the context of other Western regional capitals, like Guiyang, Kunming, Chengdu, Xian, but also Nanchang and Changsha, how does Nanning compare in wealth and sophistication?
Note that Central China is already quite poor. The GDP per capita is Anhui 5116, Jiangxi 5130, Henan 5518, Hunan 5936.

The train trip Nanning-Shanghai goes through Changsha South and Nanchang West, so if Nanning is 3rd tier rather than 2nd, are Changsha and Nanchang also 3rd tier?

Regarding that coastal region: there are now 4 daily trains Guangzhou South - Beihai via Nanning East, trip time 5:50 to 6:07, price second class 226 yuan 5 jiao, first class 272 yuan. Nothing to Fangchenggang, though - just 2 daily trains originating at Nanning East.