I have a 42 lighting big step staggered.
810hp
bravo1 -3 Imco shorys
Stelling boxes high up
Best run of 105 cold weather/low fuel
Question:
Will 6’s pick up any speed? I know the reliability with be much better.
pros and cons?

Pros- Roooost

I am no pro at drives, but curious to read answers! I would think it would slow down a bit because being bigger it must drag more in water....

More frictional losses internally sucks a bit of horsepower. Plus the need for a transmission, again, more frictional losses.

Thats an expensive upgrade, transom work, move the engines, add trans, new exhaust. and on and on......IMO the #6 will eat more hp to spin them

Current 6's and 8's do not take more hp to spin, even including a good hydraulic trans, at engine rpms above 5600 rpm. Parasitic losses are not a straight line curve and are not a simple percentage or flat number, as many people state or think. The Bravo parasitic losses go up fast at higher rpms. There is defintely more drag if not mounted high enough and the Six skeg is part of the reason too, but it is that large because it is intended to maintain control at very high mounting and running conditions.
The biggest issues, as pointed out, are related to the required engine location changes, the added weight of the Six hardware and transmissions, and is the bilge space adequate for these changes(?). There is a reason why Bravos are popular, although sometimes misapplied, when the hp goes up.

I would think 6's will slow you down. The weight, large gear case hydro drag, duel down shafts and transmission gears all come together. I am surprised you can keep brovo's together at that HP on heavy boat.....

In the now defunct Supercat Lite racing class with twin 525's, the MC6 was proven to have a faster top speed, better handling, although they did use a low hp draw SCS crash box transmission for a slight advantage over a typical sumped (BAM) in total drivetrain power loss.
As a comparison for your application, if I remember right, a typical stock (non whipple mod) 700SCI boat would run those speeds with sixes. I know that they had the NXT transmissions, which have a little lower hp loss and are about 50 lbs lighter per trans than the sumped BAM modified 72C's (roughly 100 lbs for a mod 72C and cooler). The IMCO boxes may make the drive weight comparison close, with maybe a little more added weight for the sixes depending on the exact hardware comparison (IMCO external steering, added driveshaft in the extension box, etc).
Something to consider, how much bilge length ahead of the front engine do you have before considering such a big change? The NXT transmissions on 700SCI's hold the engine location to the transom the same as Bravo's, a reason they were popular, but the BAM 72C close couple (without the special mods and transom spacer they offer to replace the NXT transmissions) will move the rear close couple engine forward about 9.5" from a standard Bravo close couple mounting. This may require the forward engine to move also, is there room?

Thank you for all your responses
I am well aware of all the work needed to make this happen. Also I do have the room to do this.
My concern is doing all this work and loosing speed.
I am aware the bravos im running with the HP is playing with fire. I’ve been running this set up for a while. Blew two uppers in five or six years. I take it easy on the boat I mostly cruise 3-3500 more than anything.
If I do this over the winter I’ll post my results
Thanks

Sixes will always be faster. The drawbacks of being heavier and trans taking more power to run is offset by a true surface piercing drive, less drag. Better prop selection, and of course reliability. On your 42 you will want them high, and you will want a big diameter prop, at least 17 inches.