It's hard to keep a tech lead in the ancient ages anyway. Techs are cheap, and once you've learned one it gets cheaper to research for everyone else. On top of that, the AI trading bonus means they tend to get sold quickly, thus spreading out the research over several civs.
To me, the real bonus of free techs from expansionist trait is in *cash*. You can sell your new techs around if you like, and gain it directly, that way. You gain it indirectly by not having to buy as many techs. And if you like, you can gain cash by resting on your laurels, turning science off (or to min) and raking in the gold while you wait for the AIs to catch up to you. Makes those massive upgrades a trivial matter, gives you gold for cash-rushing, or however you want to spend it.
Renata