Are you talking about an electrostatic precipitator (metal plates with a power supply) or an electrostatic filter (fibers with a surface charge)? Electrostatic precipitators are neat, but there are basically no standards for them and they’re not cheap to operate. For charged fibers, I see no a priori reason to expect amazing performance or to expect them to remain charged after a bath. If they met MERV 13 standards, they would say so.

In any case, if you see lots of gunk, that’s not the hard-to-filter stuff. I can say, as the proud owner of a monstrous HEPA filter with a dirt cheap noting-special MERV 8 pre filter and an utterly boring metal louver before that, all continuously collecting outsize air, the louver gets a bit gunky, the MERV 8 filter turns black after a while, and the HEPA filter is indistinguishable from brand new. This whole system replaced an older “ISO ePM1” (yes, the manufacturer conveniently forgot the number after that, but it’s MUCH higher spec than MERV 8), and the indoor air quality as measured by a little particle sensor suggested that the ePM1 filter missed about 50% of the outdoor PM2.5, whereas the new system produces air that measures zero across all particle sizes. And that ePM1 filter did a fine job of turning black :)

Get a particle counter and test your system!

P.S. the HEPA system uses less power and will cost less to operate over time because it is HUGE but has the same flow rate.