Leech Starships are, to me, essential. They pay for themselves shortly. I cannot imagine a build that is more efficient without them. But then again, I'm playing a 7.3 AI and I cannot build T3 units, though I do have tons of IV and III units from fighting my way through AI systems.
I too blob when Traversing a hostile system, but it is always a good call to go after bunched up single unit types with the unit they are weak against. Fighters and bombers will always make it back to the main group. Also if you attack a fortified position, send your fighters in first to cover the tractor beams, then send your bombers.
Remember, you can set your cruisers and starships in one group, then orbit tightly with faster units in their range craft to increase DPS. Attack move (alt+click) is your friend. One mobile repair ship in the back makes it possible to ALT attack with minimal losses, because the cruisers take so long to get there, and you end up with a stream of fighters/bombers/other constantly catching up to them. Tugs, if not tractor beamed, remove ships from tractor beam! More than one MRS is useless in my recent experience.
Let raid starships and Leeches fly ahead of the fleet on fortified positions. They take out the shields so fast it's worth it.