I have seen examples of both quite often, even for the same exact sentence, examples: おなべの中からボワっと／おなべの中からボワッと。 This is a lyric from おどるポンポコリン（Odoru Ponpokorin)、A song for ちびまる子ちゃん（Chibi Maruko Chan) In most cases, the lyrics are written with the first example, but I've also seen it as the second as well, which is more common for other words, because in other songs (also for ちびまる子ちゃん）、The lyrics were written like this: グッとグッとグッとグッと水飲む So which is more common？
There is no prescriptive rule that covers where to end the katakana section when you write mimetic words, interjections and slang words in katakana (because they are colloquialisms anyway), so we don't have the "right" answer. It mostly depends on personal interpretation: that whether you want katakana-ify the concept or the word when you do, and where is the morphological boundary in such set phrases. Multiple variations are often seen in ending -っ and adj. -い as you suggested.
As for っ-ending mimetic words, you can see a weak tendency that the more syllables before っ the less likely っ is in katakana. Especially it seems っ is always in katakana when the word has only one syllable, such as グッと or カッと. But it also takes specific prosodic constructions into account.
- ガチャガチャっと > ガチャガチャッと
- ガチャッガチャッと > ガチャッガチャっと
- ガスでパッと明るくチョっといい未来 (←!?) [Tokyo Gas's current slogan]