I’m currently shopping for a set of clubman bars, Manx replica fuel tank, rear-sets, racing seat and some other bits, with the intent of converting my old FT500 Ascot into a cafe racer. The flat bars you refer to are occasionally referred to as drag bars, or clubmans for seniors as they will shift your weight forward and down, but not as radically as clip-on’ or clubmans.
I personally prefer the forward and down riding position, particularly at speed. The crouched race riding position also generally warrants rear-set footpegs and controls to be comfortable and remain in control, but then I’m extremely flexible, more athletic then most, and have been riding for several decades.
Yes, it will radically affect the low speed handling. It pains me to say it; you really need to learn with the standard bars first, they offer far more leverage if you get the thing off balance and need to muscle it upright. Also you don’t want to go modifying your new bike right off, at least not until you have something to compare your modifications to.
Do the rider training course first if you haven’t already, if you can clean everyone else in the slow race including the instructor, then maybe you can handle low bars. …good luck if there is a Trials rider in your class