Cloaked ships have been experimented on by modders, but afaik, no successful ship has been made so far. Current "cloaking" consists of phasing out the ship much like the Vasari Scout is able to shift itself into Phase Space at will - the problem is that it is still visible to other players even when phased out.

Stealing technology, though it fits with war, may not work out so well in Sins. For instance, the Advent would have no use for stolen phase missile plans - they use exclusively energy weapons.

There already exist abilities made to slow down ship production - off the top of my head, I can remember three, two for the Traders, one for the Vasari. TEC can inhibit ship/structure construction through either the Sova's Embargo or through culturing hostile planets after upgrading Foreign Sabotage. The Vasari's Antorak has Subversion, which works by deploying recon teams to the surface of a planet to harass ship and structure construction and provide gravity well intel.

And for the record, the Vasari have copied technologies before: although they were the first to employ the use of the Phase Jump Inhibitor, they didn't design them - they found them lying around (by some even older race?) and thought they could make use of them, but couldn't reverse-engineer them. They just made exact copies with their expertise in nanotechnology.

Hmmm...like the concept but it would be difficult to steal tech from other races but still not sure if its impossible. Need to fiddle around a bit.

From a mechanical perspective this likely wouldn't work.  In most situations, you need to be very lean on tech in this game.  Until you have a mature fleet and economy, it just doesn't make sense to go for the expensive high-level techs, so the vast majority of your technology is going to be level 1 or 2, which probably would never be worth stealing in the first place. 

If anything, we need more content to encourage people to invest in higher-level tech, something that usually only happens once you've already maxed out your economic infrastructure.  This suggestion might actually discourage investing in technology by giving your enemy an inexpensive way to steal your advantage.  I don't think this would be good for the game.

Personally, I always thought the spy was stupid.  It was basically a crap shoot as to whether your opponent wasn't looking when you were sending him in.  If your oppnonent was paying attention, spies couldn't get into buildings, end of story.

It doesn't have to be all or nothing. Providing more options allows for more diverse varieties of gameplay.

If you read the initial post you will see that it says you have to PAY to upgrade your factories.  No such thing as FREE tech.  This would be more expensive as reverse engineering/creating from blueprints takes a bit of investment. 

The counter to a spy would be having a scout upgraded to see past the camouflage that is on auto attack.  The scout would defeat the sheildless, weaponless low HP spy.  The spying WOULD be used later in the game because it would be so easy to protect your empire when it is small.  When you have labs all over the map and are focusing on many other aspects of play a player might slip into your empire unnoticed and grab a tech or limit production or something.  This never was an "all" or nothing idea.  The spy was poorly implemented into red alert, but i was using that game as an example that spying in RTS has been done before.  The quality of the implementation would be up to ironclad, and from what we've seen it would be good!

As for fleets being formed that have only all the best tech, i agree that if not properly BALANCED the game would devolve into a slush that wouldn't be as clear.  Some ships should not be stealable.  Some tech should be stealable like expansion of tactical/logistics slot upgrades on planets.  Trade tech's.  Some things that make each race unique shouldn't be steal able.  It would be up to testers to balance it and i'm sure that people would be listened to in this regard as they have in the past.

they could send replicator nanites into enemy buildings to scan the technology