I think you are on the right track with the 150-grain Speer and Reloader 22. The track is using a slow burning powder and a 150-grain bullet in the .270 Winchester.
The right track for the .270 Winchester starts with a slow burning powder starting at IMR4350 or its clones, topping at with powders in the MagPro, Reloader 22 burn speed.
I load for three different rifles in .270 Winchesters. None of them are a Howa. But a load these rifles like is a near maximum charge of H4831 and 150-grain bullet. I reload a 100 or more rounds of this recipe at a time and get about 2870 fps out of it.
This load has also been proven in a friends pre-64 Model 70 Feather Weight.
The 150-grain load is my go-to load in two rifles. A third rifle likes the 150-grain load, but I have been hunting with the 140-grain Hornady BTSP in it. I have taken a couple of deer using it pushed by H4831. I had to back off on my first loading, and the result was been effective, and explosive on deer at close range.
I spent a lot of time and effort developing a load using the 140-grain bullet pushed by WW785. This load gives me better accuracy and higher velocity. I was amazed to see the airborne hoofs of a deer doing a summersault after being hit by this load at about 75-yards.
The downside is that I can no longer get WW785. My latest development effort has been with Winchester Magnum Rifle(WMR) powder, pushing the old style 140-grain Winchester Silvertip bullet. I bought a thousand of these bullets on sale. I found a near maximum charge of WMR and this bullet to be very accurate. After this effort Winchester dropped WMR.
Two powders have my attention for the .270 Winchester in the future, Reloader 22, and MagPro.
My bullet box contains, 100, 110, 130, 140, and 150 grain bullets for the .270 Winchester. Based on hunting experience if I was limited to one bullet it would be a 150-grain bullet.