I found your post in a search and since it was only a few months old I figured i'd reply and maybe someone else had some details.
I picked up a BF this week and I found the same as you, it's just a little too dark.

If you goto youtube there are a few videos (just type in batman forever pinball in search), Some have some type of led strip between the city and sky in the back drop at the top of the playfield.
I'm not sure how it is done. One of the videos is tnt amusements talking about the pin and goofing off in the beginning. Looks really good.

The other thing I noticed is that with the pin off, the backdrop looks better because the dmd glare is horrific. Couldn't find a premade filter for the larger dmd so I ordered one of the scorevu visors from marcos for it. Hopefully it will be here by tomorrow so i'll let you know.

I also ordered a GI led set from cointaker.

Thanks guys,
Mod, can you post some pics of yours and maybe a link to the fixture you used?
I'm an electrical contractor, so I normally think in terms of voltages of 120v and up. Just not sure if anyone ever connects anything 120v on a pin.

Well, turns out the GI connector on the power supply was burnt and crispy.
Most of the top of the playfield was out, roll down dividers, red light under the extra ball ramp and all the corner bulbs (which still did flash, just didn't light as GI).

Between fixing that connector, adding led's and adding a visor to block the dmd glare it looks like a whole new pinball machine.
At this point I don't know that I need to worry about more lighting.

I'm used to working on system 11's. It's was much more of a PITA to change the GI lighting to led's on this thing.