Re: RG550 mongrel?
The link is sending me an ad for a Starfield, so I can't see the 550. While it's likely from your description that the neck has been replaced, I think the word "mongrel" sounds negative - I'm not sure if you meant it negatively - but this guitar is likely still good quality.
I've had and still have tons of late 80s to mid 90s Japanese Ibanez guitars, and other than my original RG550DY, not too many have the original necks - whether I swapped them because I wanted a sharktooth neck on a DY or just got the parts or whatever the case. However, they are still extremely high quality, well playing guitars. Again, I can't see the actual ad, but I'm assuming from your post that this has just had the neck replaced with a similar era rosewood board 550/560/570 neck. So, if that's the case, then yeah, this is a guitar made from parts, but high quality parts that I'd put up against most of the Ibanez products on the market today. So, if you can live with the rosewood and its in good shape, then I'd say it's a cool guitar.