No need to change suspension...but to get the "look" - yes, you should lower.
I'm not an expert - but regarding the brakes post...the 20" wheel's are heavier and it is harder on your brakes to stop a heavier wheel - so yes, if you get 20" wheels you may increase the wear on your brakes.
Then again - if you like the look of 20" wheels (I do) - then you have to get them.
I actually just did both mods within the past 2 weeks...here is my opinion so far:
First, my Stang came with 18" Fanblades with 235/50/18's on it.
I changed the wheels/tires first...chose Foose Ascots in 20x8.5 and 20x 10. Tires are 255/35/20 on front and 295/30/20 rears; Toyo Proxes 4's.
The result was: 1) better looks (IMO) 2) better handling, probably due to lower profile tires and more rubber to the road 3) slightly harsher ride, but still acceptable to me.
I drove it that way for about a week and a half.
Just last week, I had Steeda Sport springs installed. WOW, I'm very happy with these springs! The handling is MUCH better. It looks better and believe it or not...I actually think the ride is better! The only thing I can attribute it to is that perhaps the stock springs didnt handle the extra weight of the 20's and larger tires? Not sure, but I am really happy with it now.
Couple of pics...first one is without springs, second and third ones are with springs.
I hope that helps u out!
__________________
06 GT Vert
Two-tone Platinum/Satin Silver on Black
20x8.5/20x10 Foose Ascots w/255 and 295 Toyo Prexus 4's
5Spd w/Hurst Shifter
SLP ProFlo's w/4" tips
Steeda Sport Springs
JLT II CAI l Brenspeed/Xcal2 93 octaneTune
4.10's...more to come
Keep the tire height the sam,e and you should be ok I have 20's on mine and a Ford racind handing package. The wheels look better with it lowered.
__________________
Mineral Grey 2005 GT, Ford Racing Handling Package, BBK X-Pipes, BBK shorty Headers Ceramic coated,Ford racing exhaust, CS67 20" wheels 255/35/20F 285/30/20R, SCT Prog., Lemans Stripes, Kenne Bell supercharger 10psi 490rwhp 480rwt,GT500 twin fuel pumps,Forged and Stroked block, 39lb injectors. More to come