The filters in the RSP are definitely inadequate in a strong RF environment, especially those strong FM transmitters. I have one close to me as well. You need your own, good bandpass filtering in front of it. Re: the Flex-6500, I don't know the specs on its receiver, but I would also assume that it's significantly better than the RSP. For one thing, It has a 16-bit DDC processor, where the RSP is only 12-bit. The difference is SIGNIFICANTLY better dynamic range. Read some material on SDR basics and the value of the number of bits the DDC processor samples. Look at other SDRs with 16-bit DDCs - they all cost significantly more than the RSP. However, the RSP does what it advertises to do very well. I have several 16-bit SDRs and they receive significantly better, but I still use my RSP all the time.
There are several places to get good bandpass filters. One is from SV1AFN. His are a single board kit with switchable HF filters (one filter per HF band). If I remember correctly, they're around $80 USD. Also, you have to make your own switch box with his. There's also a company, Janilabs, that's advertising a similar bandpass filter board for less, around $55 (I think....don't quote me on that, check yourself). I have never tried those, so I can't vouch for their quality. All the inexpensive bandpass filters I've seen, SV12AFNs and Janilabs have a fairly soft passband curve, so they're not going to be as good as single bandpass filters that cost 2x-3x more (e.g. from DX Engineering, etc.) However, in the long run you should be very satisfied putting any bandpass filtering in front of the RSP, even ones that are not very sharp and don't cut signals outside the band as much as more expensive ones. Just the fact that you are lowering the overall signal level should give you better SNR, and less issues the RSP is prone to (as with ALL inexpensive, 12-bit SDRs), like ghosting.
You're probably not going to take your Flex-6500 out in the field, so that's a good use for the RSP. When you're at home, why would you use the RSP instead of the Flex? You'll get better results. Finally, don't expect miracles from bandpass filters. They won't turn a 12-bit SDR into a 16-bit SDR (i.e. they won't give you Flex-6500 receive performance from your RSP), but they should make a noticeable difference.
Here are some resources for info about bandpass filters:
https://wv0h.blogspot.com/2015/01/w3nqn-filters.html (this has a link to a QST, June 1988 article about W3NQN filters - worth a read)
https://www.arraysolutions.com/index.ph ... e_=filters (Array Solutions bandpass filters, really good bandpass filters, but really expensive as well)
https://www.arraysolutions.com/w3nqn-filters-info (another page from Array Solutions - info about W3NQN filters)
http://www.k8gu.com/?page_id=531 (another reference for W3NQN filters, with a link to a good comparison of various filters)
http://www.arrl.org/reviews-listed-by-manufacturer (ARRL page with links to some of their published material on bandpass filters)
It's also not too terribly difficult to build your own bandpass filters, it's just time consuming due to winding all those toroids. Google "w3nqn bandpass filters" and you'll find lots of good info, including some pages with info on how to build your own.
Enjoy!
73 - David, AG4F