Mulling over getting rid of my 850 mighty demon and trying one of the aftermarket FI systems..... I already have 2 O2 sensor ports available in the exhaust...... Does this require a high pressure fuel system to drive the 4 injectors in the throttle body?? Is it worth switching over from a good running carb setup???

I have Holley EFI and LOVE it. Do a search on here for it, there are a few good threads. Haxby is the local expert and distributer for Holley-

I have done the FAST about 6, easy to install and not hard to fine tune also have done 3 holley's. IMO Holley has the better system - It does not come with injectors or fuel pump(s) for the upfront price of the system because they want / you want to match the app. Holley can sell you the injectors and fuel pump(s) to, but want to know all the specs beforehand. If I recall price should be about 3 to 3.5 k max for the everything but not the install included.

There is some really good aftermarket ECU's coming out on the market and few now available but you really have to know how to fine tune them, plus to really benefit the gains for some of these newer ECM's you will have to monitor each cylinder 02 readings and fine tune from there.

Yes TBI requires a high pressure fuel system. So that is something to consider first.
As mentioned above, the Holley EFI setup is preferred system because of it's wide tuning options. And yes Haxby is the guy to ask about this.

On your question of removing a good running 850 to go with TBI, I personally wouldn't spend the time or money.. Power gain would be minimal, idle quality would be the most noticeable improvement. Now if you wanted to change intakes and go with a port fuel injection setup, then I'd say it's worth the investment.

Fast is junk, I have used it in the past. I have heard great things about the holly stuff. Stay with a big name company with support, trust me.

I love my Holley HP ecm. Got it through Alex Haxby and he had me up and running in no time. Distributorless port injection using gm ls coils at that. Idles like a taxicab with a very healthy cam. Datalogging and self tuning is fantastic. Endless possibilities really.

Just curious, which Fast system are you referring to ?

Fast XFI lots of intermittent problems that I chased for years. Motors would just shut of for no reason then would restart. Bad idle and shift qualities, and I had Zul and Enzo tune the motors.I sold the motors I got so mad. There was no support from Fast at all. I think today there are better systems out there like the Holly system. I now have Pectels that Tommy from Cheif has tuned and I have had great results and HP, but they are very expensive.

I'm running a fast ez-efi on a 496 stroker and love it. I run the a/f a little fat on cruise just to cover my azz. Getting the throttle blades set properly to run the iac in a good position takes a little time up front, but it cures the shifting rpm droop most complain about.

Fast XFI lots of intermittent problems that I chased for years:

Sorry to hear that, I've run them since they were Fel Pro back in the 90s, at that time the only other choice that I remember was Accel. We've run them on boats and cars, and really can't say we had a problem. I don't think Holley got into EFI until they bought up the old Cutler system, I could be wrong. There's a lot more choices these days for sure.