Strictly speaking, under today ASCE 7, a ratio Rp/ap=9/2.5=3.6 is valid under their requirements. They said this fact in the commentary sections, where they commented that starting 2005 edition, they changed the values- valid for piping- from Rp/ap=3.5/1=3.5 to Rp/ap=9/2.5=3.6. It is true that pre-2005 edition the ap value for piping was set to 1 (why?).

By the other hand, it is hard to guess an exact value of Rp.
The component response modification factor Rp represents the energy absorption capability of the component's structure and attachments.
For example, one can think that the piping may have a high capacity to absorb the energy but the supports (attachments to structural frames) may be stiff enough reducing the overall capacity to dissipate the seismic energy.

It is my understanding that B31E addenda solved an issue considering the conservative way and ultimately reducing the tentative value Rp/ap valid for piping by a factor of 2.57 (setting it to Rp/ap=3.5/2.5=1.4 vs Rp/ap=9/2.5=3.6), but in fact following the general rule of ASCE 7 for flexible components. I wouldn't say that following calculation based on B31E or ASCE 7 is on debate, I prefer to observe that they are aspects in ASCE 7 that seems to be not specifically developed so it's better to be conservative.