From my experience, although this will likely end up another banal academic argument with the wealthy big shots posting walls of text to the contrary, a barlow will effectively reduce the light. That is, when I used a barlow, the individual subs were far dimmer at the same exposure time than the non-barlowed data.
From my experience applying barlows to increase magnification of targets like The Whirlpool Galaxy, M27, M57, etc, If you use a 2x barlow, you'll need to increase your exposure time by at least a factor of 2 to gain the same amount of info as you did when sampling at "N" seconds. IME, simply increasing total integration time (ie doubling the number of subs) doesn't cut it, but then at the time I wasn't using 300s samples either, where maybe just doubling or tripling the number of subs might do the trick...
You'll also need to pay more careful attention to guiding, as the main camera's view positional sensitivity will increase proportionately to the increased magnification brought by applying the Barlow, but I suspect you already know that.
...And there's star bloat to deal with as well, which for me meant many subs I had to toss because DSS couldn't figure out what was a star and was was just a blob of light. This was both with an inexpensive 2x Barlow and a relatively expensive corrected 2.7x Barlow.
I'd say give it a try. The whirpool will be up for weeks to come, so if it all goes to hellinahandbasket you can just image it again later via normal means.
Admittedly when I was doing so it was unguided and with ~30s subs, so "with a grain of salt" and all that...
Edited by xiando, 11 July 2018 - 03:48 PM.