I've tried three kinds of 1157 LED's... none are as bright as stock...

This is strange. I just came in from the garage trying the LED 1157. Not as bright as standard. Led going back.

It seems that the LED lights that are made as a unit - where you buy the entire turn signal or brake light as a replacement unit seem to be very bright.  However the replacement LED brake lights are not as bright as stock.

The stock brake/tail lamp spec is 21watt and 5 watt respectively, but the LED version is only 0.1 amp or 1.15 watts.  Keep in mind tho, that you can't directly equate incandescent wattage to LED wattage because LEDs are more efficient light producers.   One significant clue about the LED version is the spec says it's not for daytime use.  This means it could be hard to distinguish in the daytime against ambient daylight depending on sun angle, glare, etc.   Incandescent lamps produce light as a result of filament heat vs LEDs that do so as a by product of their chemistry.  I'm not sure how the LEDs are arranged inside inside this lamp in terms of parallel and serial current paths or how powerful they are individually, but they do make LEDs that are EXTREMELY bright; so much so that you can't look directly at them up close.

I bought one of the $10 ebay SMD style bulb, standard brightness was brighter but when you hit the brake it barely brightened at all. Drivers wouldn't really be able to tell when you were braking since there would be no significant brightness change. I was disappointed with the bulb swap and left my incandescent in.

I don't think continuous flashing is legal, or even desireable, but I think a few flashes before full-on is OK.   In fact, I have done this manually when I think a tailgater may not be able to stop in time.   On my BMW I built a triple-flash-then full-on circuit, and mounted ultra brite LEDs in my trunk to utilize it.   I had cops and hwy patrol follow me many times thru town and they never said anything.