I did the quest twice both times making the rod device and killing the unseeing eye with it.

Now i wonder if I attack the Cultists soon as I see them the beholder appears and I kill it. Is it a good way. Will I miss on XP. Will I be able to go to the other areas and do the side quests (like the forgotten temple). And will I get the shield finally? Has anyone done it?

And what if I return the rod part Gaa... as he asked me and kill the beholder as it appears then. What will I miss then? Will I get the shield?

If you attalk Gaal when you meet him, you miss out on XP. If you kill him, I'm not sure if he drops his key, but if he doesn't, then you can't get Kangaxx's arms and legs.

You do not get Gaal's key if you attack him. My first game I had a paladin, and managed to start a fight with the cultists via dialogue. The beholder appeared in the compound to defend the cultists, so I got the exp for killing it, but missed out on everything past the locked door. So yes--you miss a TON of exp by doing it the quick way.

You would miss some experience... but it's not an amount of xp you'd really neeed, especially if you're a party of six - you rack up more xp for killin monsters than you do as quest rewards except in a few quests in the game. You'd also miss some items that can be handy, like maybe some magic gauntlets and a halbard IIRC. Of course, you'd miss the story too, but it's kind of a fetch thing, once you've done it before. Frankly, in some cases the best role-playing for your character would be the direct assault (perhaps like Olorin's pally). Anything can be justified with some ability score or role playing idea!