well how accurate they are is how well there made, and how well your gun shoots and then how well you shoot
the size of the dot, the bigger is made more for faster target accusation and as such not made for more precise shooting
that doesn't mean you cannot be accurate with it
but the size of the dot determine how much of a target it covers, thus, makes being more accurate harder, as you can be off a slight amount and not notice as much really!,
adding more power, can help, 2.5 power, well to be honest that's not much of gain really for accuracy work, even if just at 50 yrds
sure it will help, but not like a 4x or more will
red dot scopes are more like scopes with a first plane retical, some what in that as power increases the dot gets bigger
But most better model dot scopes these days, have the option of many different reticles to pick from on the same scope, many have several colors too you can switch through on the scopes
many many troops are issued red dot type scopes in 4x and do very well at 100 yrds with them
but for precise shooting a regular scope and cross hair and some higher power is always better
there are many folks that shoot 1,000+ yrds with peep sights still, so, a lot comes down to the shooter and the gun , over optic many times!
My general rule is finer the aiming point the better you can shoot for smaller groups, that is if you can see the aiming point and you and your gear are up to it! bigger the aiming point the faster you can get it on target
and why so many pin's on bows have gotten smaller over the yrs, cause they allow you to aim more accuratly, but get harder to see in low light!
good luck