In the ASW config, with full fuel, weapon and stores load (2xMk46 + 22 Sonobuoy, CMs etc) the CH148 is pretty much taking off at Max GW. I think you are under estimating the weight of the ASW config. This is more or the less the with the MH-60R btw. An aircraft like the AW101 is really the size of aircraft you need to accomplish the mission with spare capacity. The RN Merlin Mk2 can be equipped with both a door mounted M134 or .50 (M3M?) while retaining the ASW kit.
The CH148 is funded, designed and will field two GPMG (C-6); one in the Cabin Door and one on the Ramp. The C-6s generally will be used for the all the previously mentioned mission sets (counter piracy, limited NEO missions, etc..) While it is technically possible to replace those weapons with miniguns or .50 cal there would be significant cost in resources to acquire, design, test and field them that really is the primary limiting factor. The way MH dets deploy you really couldn't practically deploy with two extra dedicated door gunners. If needed the crew would be adapted with the existing complement of SENSOs/TACCOs to fulfill the door gunner role when required. This is the common method amongst allies as well (USN, RN, RAN etc..).
As well , the MH community has a laundry list of capability deficiencies (which I believe improved ASuW weaponry is already on the list) that need to be addressed before it got to miniguns.
Finally, the RCN or larger RCAF has never been a big supporter of the requirement for an improved armed MH helo. For better or worse, the RCN/RCAF doesn't see it as a priority. Other the last 30+ years there have been various efforts to improve the armament of both the CH124 and CH148 that have largely failed due to lack of interest by the RCN/RCAF. At one time, this included arming the CH124 with the M134 and we even had a mock up of it. ASuW weapons were looked at for the CH124 at various times including a study to integrate the AGM-65 on the CH124. At one point, a feasibility study was conducted by industry looking at various armed helo approaches for the CH148 (this included common missiles AGM-114, AGM-65, AGM-119 etc). Everything is in the realm of possibility but none of these concept have ever reach the point where they were high enough priority to be actioned. Probably similar reasons why the CP140 was never armed with AGM-84 or 65.
Finally, (though I have been out of the loop for a couple years), in its current state the MH community doesn't have the capacity to take on any additional training, equipment or missions. Again I could be wrong but I think "treading water"/"staying afloat" would be an overly optimistic view of the current state of the community.
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