Hi Guys,
I found a SWEETER spot!
I found that after I add the light from Scasm, the fps drop more and I don't like it.
Effect still have some detrimental effect to the fps, even only one emitter.
I changed my plan to edit mipmap using good old friend, ImageTool.
My original texture is 128x128 pixel and when I use ImageTool to do dds texture, it came out 6 mipmaps.
I think the solution for the visibility range is to manually edit mipmap. My plan is editing the last mipmap to be brighter than it's normal mipmap. The texture always has black background, when it comes to last mip, it turn to be all dark.
I opened up the texture with Photoshop then saveed as another file (xxx_mip01.psd) and reduce its size by half. Then do it all over again and save as (xxx_mip02.psd) then you will have texture that reduce size to half successively. I edited the last mipmap which is xxx_mip05.psd to be brighter almost pure white for the area that supposed to be the light of taxi light.
Then I used ImageTool to open my main texture. Do DXT5 and Mipmap and save it as dds. Then use Image->Add MipMaps and choose the PSD file that you have just edited (xxx._mip05.psd) and it will be your last mipmap, replace the default one.
OMB( OMG=Oh my God but I'm buddhist so I'll use OMB= Oh my lord Budda

)
The visibility is excellent, the fps is excellent. I have to throw away my effect out now.
These are the taxi lights without effect (only texture or light) BEFORE editing mipmap.
These are the taxi lights AFTER editing the mipmap. The last mipmap stay bright. So you can see it far...far....away.
This is how mipmap was edited.