I'm not used to RA so I'm not the best to tell, but, if I'm not mistaken, unlike most other programs, RA these days indeed "overrides" AOSD definitions much like Groovy MAME does, so your best bet is asking in the RA forum how to properly configure it for what you want to do (i. e. porting a specific modeline from, say, AOSD).
Said that, what you want to do goes against any possible logic. If RA has evolved in this particular regard lately is precisely to keep the original video timings and be as accurate as possible to the emulated hardware. You're sacrificing all this (and at the cost of a +8 % speed jump, no less) just to get a full-screen display, which either, isn't supposed to be, or is easily achievable with the monitor's (TV's) geometry settings.