The point of getSupportsShadowFilter is to show if ARB_shadow is supported (this extension moves the depth-compare to the hardware), this is not directly related to hardware PCF.
Hardware PCF can be achieved by setting a FTLinear filter on the depth map (FTShadow is the same as FTLinear, but with some extras). Also, I believe one needs to use sampler2DShadow and shadow2D in the shader instead of sampler2D and tex2D.
The problem here is that ATI doesn’t seem to support ARB_shadow too well. To still get the benefit from hardware PCF on ATI, just set the filter mode to FTLinear, and use the shadow samplers in the shader. (I don’t know if its possible to use hw pcf in my shaderless shadows code.)