But a LOT of G1 characters have been retconned in ways that make them very different from their original G1 counterparts in IDW, not only characters from Japanese G1.
e.g.
+ Metroplex; the Metrotitans are like freakin' giant archangels
+ Prowl; he's always been a knob, but he wasn't so morally dubious in G1
+ Ultra Magnus; Minimus Ambus behaves differently from G1 Ultra Magnus, although he is actually living up to G1 Magnus' motto (hence my love for this character is fully deserved

).
+ Cyclonus; the original G1 character was described as completely incompassionate, but IDW Cyclonus is extremely passionate; a passionate patriot of Old Cybertron, and passionate in his love for Tailgate.
+ Galvatron; a distinct departure in origin from the standard one for G1 Galvatron (although arguably more similar to the original G1 toy's tech specs which never mentions Megatron and eludes that he is a separate character). Not so much megamaniacal, but also sees himself as a patriot serving the greater good.
+ Whirl; original G1 character was described as reckless, but he was never portrayed as being the moral vacuum that he is in IDW. I still loved that moment where Dai Atlas described Cyclonus as Whirl's friend and Whirl goes, "My what now?"

<-- I give him a really gruff voice, which works hilariously when he's using that little girl human avatar and I keep the same voice!

+ The Headmasters; no, not the freakin' Machination... characters who were Headmasters in G1 and not necessarily so in IDW; their personalities are often different. Possibly due to the absence of having merged personas with their Nebulan/human components. Fortress Maximus is a lot more volatile, Scorponok is more like who he was before he merged with Lord Zarak (who introduced a more noble element to his persona), G1 Brainstorm is the brainiac who never shuts up (The Rebirth portrayed him well) whereas his IDW counterpart is weirdly secretive -- Chromedome is arguably the most similar to his G1 counterpart in that they're both specialists in working with the minds of other Cybertronians, although G1 Chromey worked by increasing intelligence whereas IDW Chromedome works by altering memories.
+ Swerve; his G1 profile pretty much describes him as being a really awful driver, but nothing about him endlessly talking to mask his personal insecurities. In fact, Swerve's IDW persona is more similar to
G1 Bluestreak, who was described as:
"Conversations don't come easy when Blue Streak is around. His talent for incessant and often inane talk tends to drown out those close by, but at least his gregarious manner serves to lighten the otherwise grim situation of the Autobots. What is not widely known is the underlying reason for his endlesss droning: to hide the fact that he hates war." <---okay, Bluestreak hates the war because he actually dislikes conflict but he's still a very capable warrior, whereas Swerve hates the war because he's not a very competent fighter and it's just getting in the way of his true dream of owning a bar.
etc. etc. etc.
And I don't mean to say that I think the IDW characters are bad or anything, far from it, I quite like them... but what I'm trying to say is that Metal Hawk is far from unique in that his IDW counterpart is a significant departure from his original G1 namesake. And it's not
that big a departure, really. Metal Hawk has never been one who revelled in combat, the significant change is his stalwart pacifism (which the G1 version obviously never had). Dai Atlas and Star Sabre have had complete personality make overs though -- arguably both becoming the total opposites of their G1 counterparts. G1 Star Sabre was basically your goody two shoes Optimus Prime-esque Autobot leader, whereas Dai Atlas was very much the lethal enforcer.
Overlord isn't really a departure at all, IDW Overlord is more similar to Hasbro's Overlord rather than Takara's Overlord. While Takara Overlord did become an amoral character after separating from Giga and Mega, he lacked that independent will/ambition and was essentially a pawn for Devil Z and Violenjigar. Hasbro Overlord's profile on the other hand describes him as a power monger and his function is "Self Contained Nomadic Assault Machine," which essentially sounds like what a Phase Sixer is.

Then you have characters who are more accurate portrayals of their original G1 profiles than they were actually portrayed as in G1 canon. Galvatron is one example, another one being Thundercracker where IDW have
finally shown Thundercracker for who he was always meant to be; morally conflicted.
But yeah, I always try to visualise TFs have the same voices as how they sound in the cartoon whenever possible, even if their personalities vary greatly. I see them as being the same characters but existing in different parallel worlds, but they would (should) still sound the same. And it's something I've done since the 1980s; e.g. G1 cartoon and G1 comic Blaster are VERY different characters, but I always visualised G1 comic Blaster w/ his cartoon voice.