There are two questions there. One: I don't know of any cheats. Two: lag and
its converse, host advantage, easily accounts for seemingly godliness. That
and mastering the few basics of play. Aimbots are ridiculously unnecessary.
Pull a little lead on moving targets. In case it needs mentioning, use the
mouse rather than a gamepad for aiming.

Same here.  I have seen players that really are that good.   But just 3
shots?  I think if your shields are a little low, yes.   But it takes 4
headshots in a row from the pistol to drop you if your shields and
health are both 100%.  Two sniper rifle rounds to the head.   However,
if your S & H are full and you pick up an overshield, you'll survive a
Scorpion canon round :)
For a year my son trained new clan members for team competition.  Every
now and then I'd drop in on their server to see what kind of mischief I
could stir up.   I'm hell with a Hog and love playing a modded
Bloodgulch map.  Matter of fact, it was the first stable mod for Halo PC
I know of.  "Gunner Drives Hog (and shoots at the same time)" mod.   But
everyone playing on a server running that modded map needs to have the
same one.  Else, cries of "CHEATER!" come up all over :)   Some call
that a cheat, but it's really just a simple map mod.
For my son and the people he trained, they ALL prefer the pistol to any
other weapon in Halo.  It has a little zoom plus the rate of fire is
fast, it carries plenty of ammo and its accuracy is as perfect as the
sniper rifles.  You can get 4 rounds into an enemy faster than you can
fire one round and chamber another with the sniper rifle.  The pistol is
a lot smaller, much easier to run n shoot.   But practice for accuracy
is a must.
McG.

Ya got that right!
McG.