I think we'd crush the Imperium's military. Their infantry tactics are singularly terrible (they rely WAY too much on either static defense or overwhelming numbers at the cost of individual prowess aside from Stormtroopers).
So did the Russians. The Imperium have got bodies to spare.
Laser weapons cauterize their own wounds. We don't use bullets because they're super cool, we use them because a hollowpoint round will blow a hole the size of a fist through a dude, and AP rounds will cause a great hole that'll bleed badly. A lasgun won't do that.
There's a calculation going around, where it assumes that megathules is a smeerp for Megajoules; a lasgun is a point and aim weapon, unaffected by gravity, coriolis, or wind (although air pressure and moisture could cause reduction in power slightly, but not so binary as it is today). Anyway, it works out that assuming all those bits of energy get to the target (assuming a bit is lost to the environment through light and imperfect conditious), that all the energy imparted would be the equivalent of a 2 ton truck travelling at 40mph hitting you. Being hit with a bullet is like being punched, really, really, really hard. Now, even though that energy is heat, rather than kinetic, it's going to more than "cauterize", it's going to turn your insides to ash. And for comparison, I don't think that Mike Tyson imparts the same force as a 2tonner travelling at 40mile and hour. It's unclear whether or not the lasrifle even "kicks" for rapid accurate placement of follow up shots.
If someone is doped up, on khat, weed, morphine, or just pure adrenaline, a 5.56 will go through. Leave a hole in them, but with body armour, there's no explosive release which causes them holes. Body armour doesn't "stop", it just keeps your guts intact. Multiple "impacts" of a lasrifle is going to be far easier to get through the body armour as well, weakening the glue which holds the kevlar layers together, and just cracking the ceramics under the immense heat.
Their air force is big, bulky, and relies on visual range to fight. The last time our jet fighters NEEDED visual range to engage the enemy was Vietnam.
Something that has annoyed me slightly, but I must admit as an Infantryman myself, Pilots whether they be Helicopter or Fast Air all sound a little mary-sue when I speak to them, and possibly the same applies to 40K writers. On the other hand, perhaps with Alien technology, the guard equipment has been determined to account for the need to dogfight or be in visual range. I need to read Double Eagle again to double check it, but there might be more stuff in there. Anyhow, I'm fairly confident that in the case of special forces units, like Storm Troopers, or Elysian D-99, that they'd have the technology specialising in countering their own forces with equipment not necessarily available to normal troops.
Their armor is hugely ineffective- the Leman Russ' side skirts have an angle of approximately 0 degrees. That means whatever armor they have on it, the Leopard II or Merkeva's will be vastly superior thanks to the angles of the plates. The battle cannon is more of an artillery weapon than a proper tank killer.
We are unable to judge the comparisons. Chobham armour is not made publicly available, and according to source, Ceramite is handwavium composite. As for usage of weapons, IA1 has rules to give LRBT's AT rounds, much like prior to the 5th edition codex update, Vanquishers could have both AT and HE without having to purchase Beastkiller rounds. However, regardless of whatever armour we have effective, I sincerely doubt it can keep up with the kinetic force of Ordnance weapons, Lascannon's, or Melta.
The Imperial Guard only exists to soak up the damage while the Astartes or elite units move into position- without those elements, say battalion to battalion in the middle of Kentucky, they'd get wasted.
Fallacy. While this is a belief commonly held, it's not a correct one. What happens when elite units are not available? I sincerely doubt that a middle of Kentucky "Somme" would occur, and especially while we do not control the high ground, and long with it's Eyes in the Sky, we wouldn't stand a chance.
Not to mention that the Guard would not be deployed in such a location without being able to guarantee mass landings (As on Vraks), they'd likely hit Capitals and Major Centres. It's all well and good in saying "Yeah, 5th fleet is awesome", but when the 5th fleet is tucked away in the Gulf, it's no good when DC is getting buttfucked by a Basilisk.
Talking of Basilisks, never understood why it's so less powerful than a 155mm. Even the AS90's are better than that. 23mm larger round, and the only stated distance is 15km (although it could be larger due to a bigger charge and better elevation, and the figure might well have been explorative). The AS90 can hit 25km, even up to 80km but the project was terminated according to wiki, due to supply issues.
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