The Gilfer mod was changing an FET to quiet the receiver, and some caps to slow down the fast AGC a little. Compared to stock unit, mine was a little bit less noisy and was more sensitive. I had Universal add the Bournes? knobpot which was a huge help when I used to decode FDM, or whatever the term is now. That detent for the delta tune was an insane design choice, IMO. Only thing I could see it good for was the detent allowed you to put it back on center freq without looking at it. I'm not bragging, but nobody was as good as I was at getting my M-6000/7000's to print legible stuff on the monitor. I even went down to Universal once and showed a couple of the guys who had real problems tuning it in. I got to the point I could tune it without a scope, just by ear.
The stock speaker was a joke. About 10 years after I got it, the speaker began to make buzzing noises. Inside, I think it was just a cheap Mitsubishi speaker, and instead of buying one (I found some place that had like a hundred in stock for 9 bucks each), I started looking around and called up one of the places that sells Hifi speakers and gambled on one that cost about 12 bucks and was very happy with it. I stuffed the cabinet with foam and it sounded great. I eventually put the same speaker inside my NRD-525's speaker, and added a high cut filter that I found in an old magazine.
The 515 was a tough tank of a radio, mine survived without any real problems a power surge that took out two of my VCRs one day when I was gone and had forgotten to unplug it. Only thing killed on it was the meter light, which gave me an excuse to replace the bulb with an LED that came from a friend's model train parts bin. It was a little on the orange shade, but worked great and never died again.
Posting about it has me thinking about buying another one, but with the insanely bad RFI my place has, there's no point, and I still have my 525, a mint Kenwood R-1000, a mint FRG-7, an Allied SX-190, and a couple more, gathering dust. They will soon be going on Ebay. I can spend that cash very easily on other stuff.