I read it somewhere that the 500mw version with fans and stuff is the overclocked version of the 400mw, and the 400 is more stable.

Is it true ?

Also, how much actual range should I expect with the 400mw Version ?

Yes.

I got 2.5 kms omni.

you have the 400mw system ?

Yes. using it for 1.5 year.

cool so hows it like ? replace the antennas ?

And 2.5 with obstruction or LoS ? because what I read was the 400 gives 4Km LoS with stock Ant.

ALSO, are Omni the best ? what about Mushrooms ?

Yes, replace to any circular antenna.
Practically 4kms on stock antenna is a dream for this unit for me.
Please read a little more on antennae. Whether omni is best or not depends on ur requirement. Mushrooms are circular omni which is good .

so while buying the if i ask the seller to directly give me mushrooms instead of the stock, is it fine ?

i got myself a new boscam 5.8 ghz 400mw txrx fpv system

and i have a few queries about voltage use on tx unit

using 7v vs 12v battery for powering tx

is there any difference of range and quality of transmission when usng 7v or 12 v battery

which is should i use 12v or 7v

Supply voltage doesn't affect range. The step down provides 5v to the Rf module.

thanks

Price for that??? The Upgraded One??
Regards,
Aman.