Do not respond to hand-holding posts. It clutters up threads.
1-Which is better "Spellstone" or "Firestone" , I'm nearly 1000 haste ?
Spellstone for demonology, always. Simulationcraft says so, even for haste levels much higher than yours.
2-Though I have a not bad gear, I always feel my dps isn't high enough, I'm following the rotation... etc but no use, still kinda low, any recommendations ?
First, make sure you really know the correct rotation, it's in the OP so I won't repeat it here. As demo the most important part of the fight is <35% when you spam soul fire. Make sure you have a meta-cd left for this phase and time your trinkets so you can use them during this part of the fight.
Other then this, well geared locks have high haste these days, meaning that you have to cast fast to get everything out of your character, latency and 'slow player syndrome' are a common way to loose alot of dps. Anything to lower your latency can help, and training a bit on the practice dummies never hurt anyone (exept the dummies)
3-I tried using macros for "SB, pet attack, Demonic Empowerment" together, and the result was some drop in dps.
Remember to use it with SB, incinerate, soulfire and, if you like, even your immolate and corruption. If this is a dps decrease you're just doing something wrong, since it casts spells just like you would without the macro.
4- Any recommendation or advice I should follow to enhance my performance concerning "Gear. Stats, Build" ?
Spec looks good. I'd recommend getting rid of that pvp trinket, even the frost emblem trinkets are better than that one. (I'm not sure if you use it in your dps spec, it was equippen when i checked armory)