Clearly you do not understand how this would impact both aspects of the spectrum (for Arsenal) negatively. Some of those are nerfs for PvE yet clearly a buff for PvP. Not only that, you are wanting your arsenal to have the ability to NOT be able to be locked down which is poor planning. Yes, mercs need a buff of some sort for mobility in PvP, but not to the extent of making them spamming Tracer Missiles while strafing.
Lets just pick at the Tracer Missile becoming instant but doing 15% less damage.
The GCD on instants is 1.5sec. With a cast, your GCD reflects the cast time. Therefore, my 1.4s Tracer (with CR up) will be able to be spammed every 1.4s without a damage reduction.
You are saying to increase the time inbetween Tracers by .1s and reduce the damage by 15%.
I will add I do agree with the Heatseeker buff. It does little damage for our end tier ability.
Tracer Missile already hits easily for 2.5-3k. for a low heat, spammable set-up ability that damage is quite high.
take 15% away from Tracer Missile, and you are looking at 2.2-2.5k damage per missile. over 3 missiles, that is a loss of ~900-1500 potential damage.
heatseeker hits for 4.5-5k. increase that by 15%, and it goes to 5.2k-5.8k. it would likely be reasonable to hit 6k from time to time if you are optimized for power stacking
so you are trading ~900-1500 damage on the front end, for ~800-1000 damage on the backend. a potential loss of 100-500 overall damage is minimal, which is the impact that i said it would have in PvE.
and can i ask why youre taking critical reactions in your build? you need to spend 7 points in combat medic in order to get that. Arsenal is a DPS class, you could be getting better benefits for that many points than .1s off your TM cast time. not to mention that .1s is literally going to change nothing.
might i add that overall, instant TM/PS would likely benefit PvE play. there are many fight mechanics that require players to stay mobile for extended periods. instead of ending up spamming rapid shots, you will still be able to use tracer missile and/or power shot. overall, i think that would be a benefit for PvE
edit: i completely read over your comment about how Mercs would not be able to be "locked down". being "locked down" should not be a balance mechanism. Arsenal Merc is nearly impossible to play in a mildly competitive PvP scenario. The number of ways to have casts interrupted in this game is extensive, and Merc needs a way to maintain its DPS output. Merc would still be able to be "locked down" the same way as any other class: CC, snare/roots, group focus fire.