Shaders are more expensive than simply setting a blend mode, so I don't think a special Mode7Additive effect is going to improve anything.
If you've made all the improvements to the Mode7 effect I suggested (including separate shaders to remove the conditional), then I think it should be about as fast as normal rendering. The bottleneck is normally the number of texture lookups (normal rendering does one, but a blur might do ~10, requiring 10 times as many texture reads), conditionals (especially if unpredictable, which can make entire groups of cores stall if one mispredict occurs), or loops. Mode7 does one lookup, then some branchless math with no loops. I think in some cases the math is done "for free", because even if it weren't there the core would have spent the same amount of time stalling waiting for the next memory read to come through.
The only thing to worry about IMO is the fact you have two more layers which will be using their own texture. After the layer renders it then has to copy the canvas-sized texture for that layer to the screen. It's the type of thing that on mobile will hit your fillrate hard and possibly cause poor performance. However if you're targeting desktops/consoles you probably have more fillrate than you could know what to do with (especially on high end desktop graphics cards, you will probably sooner run out of CPU power trying to issue rendering commands than run out of fillrate). So fillrate is probably not a major concern there either.
So basically I wouldn't worry about it, it's a good way to get the effect working with different blend modes. Something you could do to help reduce the fillrate: if any of the layers are empty - or even if they have content, but it is offscreen - set them invisible. A visible but empty layer using an effect still takes fillrate (just to process a screen of transparent pixels); if it's invisible, it's entirely skipped, saving any rendering cost. So then you only pay the GPU cost when it's necessary, so e.g. if there are no FX under the ships then you don't spend any time at all processing shaders or using fillrate.
I wonder if the engine could skip layers automatically if they are empty. I might try experimenting with that in the next beta.