[SuedodeuS]

I like the concept, although as has been mentioned the speed required is problematic. If I'm not mistaken, however, what you seem to want is FTL travel that requires a long STL "warm-up" before it works. Here's another idea - subspace-braking.

Essentially, researchers not only discover subspace, they find that it's possible to dump kinetic energy into it - more specifically, the velocity part of the equation. This is used for "subspace-braking," wherein a vessel rapidly dumps its kinetic energy into subspace in order to rapidly slow down without having to use precious fuel. It is theorized - and eventually proven - that dumping enough kinetic energy at one time can create a tunnel into subspace, and that an object can enter this temporary tunnel. Once in subspace, distances are greatly lessened (provided you did it right), and it is found that it's actually fairly easy to break out of subspace - it's getting in that's the problem (you could have particles that get out of subspace on their own be how researchers find it initially).

So, how much kinetic energy is enough? That depends on the mass of the object that needs to pass through. The best way to figure this out is to determine how long you want the typical jump-capable ship to take to get up to speed. Once you know that, find out how fast it would be going at that time, and thus how much kinetic energy it has, and voila, you've got it. The way the amount of KE necessary scales with mass depends on what relationship you want between jump-efficiency and size. If you want it equal at (essentially) all SM's, it scales directly with mass. If you want small ships to be more efficient (which I doubt is the case), it scales with the square (or cube, or greater) of mass; if you want larger ships to be more efficient, it scales with the square root (or cube root, or lesser) of mass.

This could have some big impacts on the way space travel and combat works. First off, you probably aren't going to have much interstellar "adventure cargo" (that is, stuff that you buy low one place and hope to sell high elsewhere). With the deliberate nature of FTL, you're going to be planning all travel well in advance. If the efficiency scales in favor of more mass, FTL ships are going to pack on as much cargo as possible before a jump. Heck, if you scale it enough in favor, you might end up with the oddity of ships loading themselves up with worthless scrap in order to be able to enter subspace faster and cheaper. Such scrap would likely be jettisoned while in subspace, which could lead to any number of problems - subspace junk that damages other jumping ships, or said junk re-entering normal space and damaging satellites, colonies, etc. before it can be dealt with. STL fuel costs per mission would likely drop - as it stands now, a good deal of a craft's fuel has to be burned in order to slow down at the destination. Using subspace-braking, you simply dump KE into subspace, allowing you to slow down more quickly and without burning fuel. This would also be useful in combat, particularly if you could vector* the K-dump. If you could also redirect (with loss) the KE with this method, you could also end up with some very impressive astrabatics.

Subspace-braking could also be used to justify KE-only Force Shields. This would cause most projectile weapons (although it wouldn't work on lasers, and probably not very well on particle beams) to be less useful, as some of their KE would be dumped just before reaching the target. Indeed, planet-killers and the like would be useless - with such extreme KE, a dump causes them to jump into subspace. Of course, I suspect you don't want too cinematic of a campaign, so only the direct use of subspace-braking (that is, simply slowing down) and its use in FTL travel would be available.

*I feel this warrants a bit more discussion. What I mean here is that, if you are traveling (in respect to some arbitrary point in space) forward at 100 m/s and left at 10 m/s, you could vector the K-dump to drop energy unequally from each - so you could drop to 98 m/s forward, 1 m/s left; or even 100 m/s forward, no left movement. If you can also redirect the KE, you could change the above situation such that instead of going 100/10, you start going 80/20. This might even go so far as to give you maneuverability on par with cinematic fighter spacecraft. You can also allow a ship to drop mass (by jettisoning cargo and the like) while sapping away its KE to move faster. This can also make for interesting munitions - large missiles that use this redirection to stop dead and "fire" a much smaller projectile at a ridiculously high speed.