My brother and his best friend are, respectively, ISTP and ESFP.
In some ways they are like twins.
They are both action-oriented, observant, always open to new experiences. They are both optimistic, present-oriented, and spontaneous. Both are pragmatic and hands-on in approach. They both excel at their respective interests - science for the ISTP; language for the ESFP - but they also both prize their leisure time. Both are very independent and willing to take risks in the name of payoff. Both are naturally gifted at leveraging exactly how far they can push something to get the best result. Both are generous and freedom-loving. Both are devilishly charming.
I think it is fair to group the types together, though no better than any other equivalent grouping. ISTP could just as easily be grouped with INTP and ESFP with ENFP, or ISTP with ISTJ and ESFP with ESFJ.
The point of the system is to have equivalent multidirectional dichotomies, after all.
I also think it speaks to the lucid perception, open-mindedness, easygoingness, and pragmatism of ESFPs, how @Jeffster responded to this thread.