If you're upping from 23 to 25 then YOU WILL see and feel the difference. According to Whiteline http://www.whiteline.com.au/docs/bulletins/010barup.pdf changing from 23 to 25 will yield a 40% increase in stiffeness. Also upping the front bar will increase understeer by +1, but will let you hold the grip longer BEFORE the understeer by +2. So for me that's a good deal. Also adding rear bar to the mix, changes situation dramaticaly. Remember, that one of the reasons for understeer is roll on the front, causd by rolling onto the outer edge of the outside tire, the one that is suppose to hold the most. If you keep it flatter longer, then you will have substantially more grip, until finaly it will be to much and you will end up understeering. That of course does not apply, if your driving on some ****ty tires LOL. Tires DO change a whole lot too. When I slapped my Potenzas Re01-R's on, I noticed a lot more roll (already on H&R springs), since the tires are gripping substancially more.The stock front swaybar sizes are GLS 22.8mm and GT 23.2mm (according to Jay).
I don't think you'll see much of a difference with less than a 1mm change.