What can I say, this makes no sense to me. The fault in the Enforcer 5D is the heatsinks, not the speed.
Getz,
I had posted this before, and I really think it outlines exactly the faults with the speed increase, sure, the heatsinks are a flub too, but personally the engine boost is what kills the design for me.
To me the Enforcer really loses a lot of its flavor after the introduction of 3050 tech and all of the new units leading up to the year 3069. Frankly after this point there are many better units than a 4R Enforcer, but it is still a solid design for a militia force. The 5D is in my opinion a worse version of the 4R. It does some things right, but it plugs in an XL engine that doesn't allow the mech to move fast enough to offset its new vulnerability. Yes, you might be able to stick me in with them STD engine cartel guys... but frankly STD engines make sense for "line" units. Which is in my opinion, what the Enforcer is intended to be. The Enforcer III proposes some unique new variants that I very much like, but still suffers from that XL engine. On front line medium designs, XL's aren't my cup of tea. Where an XL shines is when it gives a mech a very high level of maneuverability for its weight. 5/8/5 on a medium in 3069 doesn't fit the bill for "high level of maneuverability for its weight".
I make some exceptions to this rule for medium "line" designs that just outright work with XL engines, such as the Lynx, Bushwacker, Grim Reaper, or Shockwave in later eras. All of these designs bring something the "upgraded" Enforcer just doesn't bring to the table, raw damage output. In each case, these mechs move as fast as the upgraded Enforcer, are just as survivable (or more so) and can deal twice the amount of damage or more from good range. Allowing them to greatly contribute to the fight before they go down.
With all of this in mind, the Enforcer really only brings two things to the table in 3050+ games.
1. Cheaper, lighter playmate for heavier nasty units that need a little buddy
2. Member of a dedicated "flanker" lance that is going to use its decent firepower/speed ratio to outmaneuver other units
As stated by Mattlov, the Enforcer is the ultimate lance mate.