The timing constraints of vinyl had nothing to do with Karajan's tempi. In the original release some of the LP sides were very short as there were no overtures used as fillers - side 2 of the LP containing Symphony No. 4, containing only the third and fourth movements, was particularly short and the 'running off' groove took up a large part of the LP's radius. It was the same with the 1970s set - no couplings.
I agree with you that this remains the most compelling of his four audio sets, although when I listened again recently to the second of his three DG sets (the 1970s ADD recordings) I realised that I had forgotten how fine much of that set is - especially Symphony No. 7.