Their tank armour is quite a bit thicker than ours as well. For reference a Chimera has around 100mm of armour, comparatively modern tanks are around 140mm iirc. Leman Russ's have around 150mm of thick ceramite and plasteel so in frontal combat there isnt much we can do.

That being said they dont use ERA blocks and from what i can tell dont use compound armour either unless its layers of Plasteel and Ceramite. So theoretically something like a Shaped Charge or a HEAT explosive could defeat the armour on Chimera's while Javelins and other vertical launchers can hit the very vulnerable engine blocks of the IG machinery.

Soldier wise their men have flak jackets but very little else in the form of body protection. Their Lasguns likewise are very bare with maybe a bayonet and no sights to aim with. So in terms of long range engagements human v human the modern military doctrine will be capable of dismantling the IG formations which rely on a very Blitzkrieg-esque style of battle. Just hit from long range, kite then fall back. Given the limitations of IG tech we can hit then escape while they fumble about given how large their vehicles are and how poorly they are equipped.

The Lasguns themselves while extremely powerful i.e. .50cal levels of strength, it doesnt really matter if they cant see their enemies while their enemies using scopes can see them.

However if the IG come with shocktroops and carapace armour this might switch it around but im not very hopeful given how modern tech is pretty much a tit for tat for Imperial tech at this point in our evolution.

Their fighter aircraft might be able to beat ours out but i dont really have enough information to make a judgement call on that one.

so tl;dr imperial tech doesnt really matter as much in this fight and modern tactics are specifically design to counter IG ones. Rather even fight.