alexban,
A very good post with excellent responses, and no-one going off at ridiculous tangents, I hope that this is not one of them.
What you are essentially arguing alex, is the relative merits of two different techniques, i.e. use of speeds for your Actual TOW (48T) or for that achievable at limiting conditions (50T at +50°C). Everything in aircraft performance is a compromise, lose a little here, for (hopefully) gain a little more somewhere else. There are benefits and merits to both techniques, both are safe, and both are legal provided that it is allowed by the AFM.
To add my tuppence worth to the discussion, and to address your questions regarding the use of higher V speeds -
(1) "Is it unsafe" - No, More safe for the continued Takeoff, Less safe for the rejected Takeoff, but still within normal parameters of safety. As several posters have alluded, if the aircraft can stop from the same V1 at 53T, it certainly can at 48T, and with ASDA in reserve. It is not, as you put it, "an invitation for overrun". For the continued Takeoff (i) Wind Shear protection is considerably enhanced at the higher V2 [On pre-reduced Thrust aircraft it was common practice to use the V speeds for the highest permissible TOW when wind shear was possible], and (ii) OEI climb gradient will be improved, see the next item for this.
(2) "Is it like doing Improved Climb?" - Yes, you are doing a 'de-facto' improved climb. V2min may be as low as 1.13 Vs, where 2nd segment climb is obviously acceptable, but well below the optimum available at speeds closer to Vmd. There is no upper limit prescribed for V2, but IN MY EXPERIENCE I have not encountered a useful 'Improved Climb' V2 above 1.38 Vs. To put it back into simple terms, if the aircraft can achieve the requisite OEI gradients at the V2 for 53T, it will certainly exceed them at 48T, even if, in extreme circumstances, that V2 for the actual weight was above Vmd (A pretty extreme case).
The summary of the above waffle is -
Accelerate Stop performance is degraded, but still within normal safety limits.
Continued Takeoff OEI performance is improved.
Wind Shear protection is improved.
Both techniques are acceptable and safe. Mutt, (a highly respected Performance contributor) I observe, specifies the use of Assumed Temperature for his company, but then uses FMC V speeds for Takeoff, that is, V speeds for actual weight. Some operators ignore FMC speeds, and directly use those for the Assumed Temperature (and it's maximum TOW) straight from the Airport Analysis. My own company has a compromise, use the FMC speeds if within 3 knots of the Airport Analysis speeds, and use the latter if outside the 3 knot limit for any one of the 3 speeds.
One comment buried within the responses makes me acutely uncomfortable, that of using the Vr and V2 for the higher weight, but using the V1 for the lower actual weight. Good for the rejected Takeoff, but you're walking straight into the Devil's playground in the continued Takeoff case. There is a clear relationship within any set of V1, Vr, and V2's, the primary one being the capability to accelerate from V1 to V2 at 35 feet by the end of the TODA with one engine failed. The V1/Vr/V2 split in the AFM allows for this, but, if you're introducing a larger Delta V than certified between V1 and Vr, NOW you're inviting an extreme overrun. If you doubt this, take a look at the Wet Runway performance where the V1/Vr Delta is often much wider. Achievable?, YES, but only at a reduced weight.
FE Hoppy, If your aircraft wants to become airborne before V1, you're a pilot, you know how to stop it. It's not a problem.
Fly Safe,
Old Smokey