Dobbsy I wouldn't recommend a BP effect for the demolisher cannon. Its not performing a wide area suppression role like typical arty, nor is its gun that big that its like a 500lb+ bomb going off and hitting everything in a large area. Rather its more likely to be used in reducing obstacles and strong points through demolition or shaped charge rounds.
I think MW is justified, as morsla states, it has the Highest St available in 40k, it is AP2 which means marines and terminators armour is bypassed (though terminators have their backup invulnerable save) whilst terminators can pretty much walk through battle cannon fire (as its AP3). The only time a multimelta is better at penetration is when the melta weapon is within half range, thought clearly that's getting down in the weeds and be like saying in epic that a multi melta would be MW at 0-7.5cm and AT at over that to 15cm... probably a bit silly!
There are few other weapons in 40k that have the power of the demolisher, for comparrison the power weapons that typically grant a MW hit in CC are less powerful than a demolisher cannon.
If you were going to do it though i wouldn't bother with Ignore cover. The MW ability sort of covers this anyway as there is no cover save against the hit, just the -1 to hit. I say that for these reasons. Ignore cover should be for those weapons that can somehow bypass the cover or flow around it, (eg flamers, tau smart missiles, some alien psychic attack etc, even the thermobaric style bombard shells. I see the Demolisher as something that just blows its way through the armour. So the fact that cover obscures you makes it harder for the vindicator to identify your exact position (-1 to hit) but wherever it hits the cover means little due to huge HE round and shockwave etc.
At the end of the day its a game so do what works for you but I think there is certainly justification for MW if 40k is to be used as a comparison. SM/TL era vindicators were also a -3 or -4 save IIRC so there is precedence within the epic community for these things to be MW in effect.
As for the plasma cannons, IIRC when we discussed these in the earlier days of EpicA the AP4/AT4 slow fire came about because back in 2nd ed a heavy plasma cannon could fire at max charge, then take a turn to recharge (hence slow fire). Firing at Max power, the heavy plasma gun was more powerful than any other man-portable heavy weapon, though the multimelta could theoretically cause more damage.
Comparing the current hvy plasma gun to say a missile launcher, the krak missile has a slightly better chance to cause a penetrating or glancing hit, whilst the plasma cannon due to its AP2 is more likely to cause an kill/explosion if it penetrates.
So what? well I'd say in a current 40k to EpicA conversion it would be on par with a missile launcher for AT. It is worse than a lascannon. Against infantry it is better than a missile launcher, same chance to hit but greater chance to kill and bypass armour - so what? well AP4+ is probably appropriate but then that puts in on par with a battlecannon which in 40k is generally much better at killing off a squad. Call it AP4.5+ and round back up to AP5+?
The other consideration is that the standard plasma gun is 15cm AP5+/AT5+. I like the AP5/AT5 stat line for infantry weapons as its a bit of mix between the old and the new, but removes the accounting required from slow fire.
Given the consideration that the Baneblade and Vindicator are considered sub par and both have had threads over the years to address their performance, MW on the demolisher cannon would probably help both out. The only other user i'm aware of off the top of my head is the LR demolisher. It can be pretty handy, especially with the 2x AP4/AT4 attacks from its sponsons on top of the main gun and lascannon. Toning back the plasma cannons to balance the MW boost would probably keep this unit at is present "power level" for want of a better term. Though that is something you would have to bring up with the IG guys - who are probably wanting the same thing but avoiding it for fear of upsetting the marine guys

Still we can trial it, would you recommend the same points for swapping the main gun from:
30cm AP3+/AT4+ IC
to
30cm MW4+
?