In my games, we've come to the conclusion that staple weaponry of 40k such as the Plasma Gun and the Power Sword are way underpowered, based on what they are able to do in the boardgame. In Rogue Trader, the Plasma Gun is marginally better than the Hellgun. A weapon which fires the heat of a sun in peoples faces.

This same problem are applied to some other weaponry such as the Power Sword. The weapon is stated both in fluff and on the tabletop representation that it can cut open Terminator Armours like can openers. But with a meager 5 in penetration, it makes it even weaker than a plasma gun.

What's your opinion on these weapons, and are there any home-made fixes to this already in circulation?

Few things to point out: The plasma gun is marginally better than the hellgun when fired on the safe setting. At 2d10+7 with a Pen of 8, you'll find a plasma gun quite capable of punching through power armour. Still doesn't really match up with the tabletop, but most things don't.

That said, there are house rules kicking around. http://darkreign40k.com/drjoomla/index.php/armoury/weapons-and-armour/907-rewrite-of-plasma-weaponslights-stubber-rules