lobing would only be a concern if the driver were playing the same passband, like you I haven't noticed any adverse effects of horizontal mounting.I've read (not confirmed) that horizontally mounting a mid/tweeter together isn't a good idea due to lobing. In my car, I didn't really notice a problem with that. I'll be trying vertical alignment (mid on bottom, tweeter above it) this next go-round to see what if I notice a real difference. $.02
TA won't matter at all for the tweeters really, level matching is much more important for high frequencies.I'd like to keep the mid and tweeter as close as possible because I'll be splitting T/A to both of them and I don't want it to sound different on each driver.
I would suggest a 3" fullrange (the peerless is excellent) with a 3/4" dome crossed over at 5-6khz. I've A/B many GREAT 6.5"/1" combos with the conventional ~3khz xover and I can't tell you how much more coherent a good fullrange sounds taking over that lower treble and eliminating that xover in the crtical range... i can't see myself going back to a conventional 2way anymore.