actually, its like this;- tiagra=4xxx 105=5xxx, ultegra=6xxx dura-ace=7xxx , in the mtb world deore/stx=5xx, lx/slx/hone=6xx, xt=7xx, saint =8xx and xtr=9xx. they havent yet released an ultimate road bike groupset, they could do it any time, it took nearly 10 years to deliver xtr, so who knows?
thing is tho, shimano kit road bike kit is such good quality now that none of it is considered tat, its all good.