I know that there are more Cos/Sin function than just Cos()..
..maybe Cos256 is based on something different..
Nope, checked DelphiX?¢_Ts source, Cos256 simply uses a lookup table precalculated with
for i:=0 to 255 do
CosinTable[i] := Cos((i/256)*2*PI);

As I said the angle is in radians, so changing it by 1 is rotating by a almost 60 degrees, change the angle in much smaller values like one degree (that is pi/180).