There's two separate issues here - one, that Great Scientists are overpowered, especially given exploits in the technology tree, and two, your example case.
I agree that being able to pop multiple great scientists in the same turn means that you can rapidly achieve technologies that can shift game balance dramatically. A typically exploited weakness is going from Steel & Physics to Rifles in 1 turn (normally around turn 80-90 depending on GL/PT) or to Fertilizer (if you don't have any close enemies). These could easily be addressed with the X-turn wait period suggested earlier. I like this as it's a simple change, and not likely to screw with the balance of the game (which seems nicely balanced at this stage apart from HG and _maybe_ the iroquois rush on small maps). There could be other "complicated" rules you could add on, but imposing a cool-down period would be a step in the right direction.
It still means you can do a 4-tech with an appropriately timed OC / Rationalism / GS, but it's a one off event that gives a science player the edge for a period of time. The game can't be balanced at all stages - e.g. a science / wonder player very vulnerable to early rushes (sorry to all those reading who have been rushed by me of late). So I am not so against that - it's also available to all players (unlike wonder races or UU/UA/UB).
As to the second point, I agree with the other people like Tab that are not so keen on your example. To get Mech Inf with 5 Free Tech pops ready to go, you need the almost the whole ren tree, excluding only Fertilizer, Metallurgy and Rifling. Consider the tech difference between Art Rush and getting Sci Theory / Military Science. The two late techs (Arch/Sci Theory and Fertilizer/Rifiling) balance out. So the difference in tech is the whole naval branch (1635 beakers) vs Metallurgy + Dynamite (2344). So it's almost the same total cost in bulbs, however, you need to take into account your opponent doesn't need to "save" 5 free techs. They can probably bulb at least 2-3 of the techs to get to Art a good 10-15 turns before you are ready to pop. Even then, once you have Electronics, you don't have any upgrade path, so you either have to spend 600+ gold per unit or hard build them (251 prod). Not to mention you've been paying maintenance on those GS for a quite a few turns by then.
Rifles and Artillery have great upgrade paths, so you need to be able to deal with Rifle Rushes from about turn 90 or so, and then at least 15 or so turns of your opponent having Artillery / Rifles. Calvary might be your solution, but mounted units are still relatively underpowered.
(I realize there's some gains from observatory / ocean buildings depending on city location, but fertilizer is pretty powerful also)
All that said, I am not at the same skill level as you though, so you'd get a chance to wipe me out with your Mech Inf
Jump to electronics is a nice follow up to Dynamite though (I normally go to Combustion then Nukes with the OC/Rationalism approach, but that's very resource dependent). Also, if you are way ahead, mopping up the other players with Mech Inf is much faster than Artillery / Rifles (especially when taking out that annoying Great Wall player...)
