Just to see for myself, I made a F2P account on Tuesday and made a Smug. I did all the quests on ORD and got all three crons and all of the Codex.
I received two of the XP buffs as quest rewards. I used one towards the end when I soloed Sav Island once I got Corso. I spent a little extra time killing elites and downed the Champion a couple of times. And got to level 10. Some extra grind, yes, but only about 10 minutes' worth.
Went to fleet got AC and was sure to grab all the Codex from the Crew Skills area (plus the little quest). This put me half-way to 11.
Ran Esseles and used my second XP boost then. When that was done, I did one PvP match. I was then 12.
Cleared out Coruscant (did all the quests, Heroics, and bonus stages crons/codex) and finished at 15. Got my ship and did all the space missions I could. Went back to Tython for the crons/codex.
Then last night I queued for Hammer. I was 16 and some when I finished. When on to Taris. I had a couple more of the XP items so I pretty much had the buff on the entire planet. Cleared it out (including bonus series) and am now 20. Right where I should be.
I admit I did some extra mob killing here and there. And I know the quests by heart, so I was able to do them quickly and maximize my time with the buff on. But I still don't think it's as bad as some are making it out to be.
If I was to continue, I'd probably skip Athiss and save my third FP for Mando. And do remaining pvp matches after Nar Shaddaa.
Then the following week, skip Cad, and save my three FPs to run T5 twice and Maelstrom Prison once. Then the third week, skip CWG and do RR three times.