From my experience, Mito vs GT86 is usually lots of fun. With the new Japanese car pack, it seems that the Abarth 500 Assetto Corse and MX-5 Cup are 2 race cars that are similar in lap time.
Below is a list I made that shows all the mixed cars combinations that have been run at RaceDepartment's AC club races (mostly road cars), or are combination I've seen on pub servers:
McLaren F1 GTR vs McLaren P1 @ Silverstone
Lotus 2-11 vs KTM X-bow @ Vallelunga
M3E30 vs GT86 vs Mito vs Abarth500S1 @ Zandvoort
GT86 vs Mito vs Abarth500S1 @ Nurburgring sprint
GT86 vs Mito vs Abarth500S1 @ Silverstone national
Mito vs Abarth 500 @ Magione
Mito vs Alfa GTA vs Mito @ Spa
Miura vs Abarth500 @ Barcelona Moto
2-11 vs X-bow @ Norisring
4C vs M3E92 @ Vallelunga
599 vs Zonda @ Monza
4C vs M3E92 vs Exige 240R @ Blackwood Reloaded
Lotus 49 vs McLaren F1 @ Mugello
Abarth500 vs Alfa GTA @ Mugello
C9 vs LaFerrari vs P1 @ Silverstone International
C9 vs 599 vs Zonda R @ Spa
F1 GTR vs P1 @ Silverstone
P1 vs LaFerrari @ Nordschleife
M3E92 vs Evora S vs Exige 240R @ Vallelunga
599 vs Zonda R @ Silverstone
1M vs Evora S @ Vallelunga Club
Xbow vs 2-11 @ Imola
Xbow vs 2-11 @ Aosta
Xbow vs 2-11 @ Mugello
312T vs Evora GX @ Mugello
599 vs Zonda @ Gentrack1
599 vs Zonda @ Blackwood
599 vs Zonda @ Silverstone International
599 vs Zonda @ Monza 66
Formula Abarth vs Evora GTC @ Nordschleife
599 vs Zonda @ Monza
1MS3 vs M3E30Drift vs M3E92Drift vs M3E92S1 vs Z4E89Drift vs EliseSCS2 vs EvoraS vs Exige240R vs ExigeScura vs ExigeSRoadster @ Vallelunga Club
458 vs F40 vs Xbow vs 2-11 vs MP412C @ Mugello
458 vs M3E30GrA @ Magione
M3E30Drift vs M3E92S1 vs EvoraSS2 @ Vallelunga
Exige S Roadster vs Exige Sucra @ Silverstone International
M3E92Drift(Mouse) vs M3E30Drift(Cat) @ Monza
Supposedly different cars get similar laptimes with these combos (I tried Abarth 500 vs Miura at Catalunya Moto it was great fun). Hopefully this could be helpful.